Allow user to register actions without parameters

[Kami]

Previously schema validation would fail. If action contains no parameters it should also not contain parameters attribute in the metadata file.

Spotted by @jfryman

2015-01-30 07:57:31,153 DEBUG [-] Loading action from /opt/stackstorm/packs/rackspace/actions/list_vm_images.yaml.
2015-01-30 07:57:31,161 ERROR [-] Unable to register action: /opt/stackstorm/packs/rackspace/actions/list_vm_images.yaml
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/st2actions/bootstrap/actionsregistrar.py", line 121, in _register_actions_from_pack
    self._register_action(pack, action)
  File "/usr/lib/python2.7/dist-packages/st2actions/bootstrap/actionsregistrar.py", line 97, in _register_action
    action_api = ActionAPI(**content)
  File "/usr/lib/python2.7/dist-packages/st2common/models/api/action.py", line 172, in __init__
    jsonschema.validate(kw, self.schema, util_schema.get_validator())
  File "/usr/local/lib/python2.7/dist-packages/jsonschema/validators.py", line 428, in validate
    cls(schema, *args, **kwargs).validate(instance)
  File "/usr/local/lib/python2.7/dist-packages/jsonschema/validators.py", line 117, in validate
    raise error
ValidationError: None is not of type 'object'

Failed validating 'type' in schema['properties']['parameters']:
    {'description': 'Input parameters for the action.',
     'patternProperties': {'^\\w+$': {u'$schema': u'http://json-schema.org/draft-04/schema#',
                                      u'additionalProperties': False,
                                      u'default': {},
                                      u'definitions': {u'positiveInteger': {u'minimum': 0,
                                                                            u'type': u'integer'},
                                                       u'positiveIntegerDefault0': {u'allOf': [{u'$ref': u'#/definitions/positiveInteger'},
                                                                                               {u'default': 0}]},
                                                       u'schemaArray': {u'items': {u'$ref': u'#'},
                                                                        u'minItems': 1,
                                                                        u'type': u'array'},
                                                       u'simpleTypes': {u'enum': [u'array',
                                                                                  u'boolean',
                                                                                  u'integer',
                                                                                  u'null',
                                                                                  u'number',
                                                                                  u'object',
                                                                                  u'string']},
                                                       u'stringArray': {u'items': {u'type': u'string'},
                                                                        u'minItems': 1,
                                                                        u'type': u'array',
                                                                        u'uniqueItems': True}},
                                      u'dependencies': {u'exclusiveMaximum': [u'maximum'],
                                                        u'exclusiveMinimum': [u'minimum']},
                                      u'description': u'Core schema meta-schema',
                                      u'id': u'http://json-schema.org/draft-04/schema#',
                                      u'properties': {u'$schema': {u'format': u'uri',
                                                                   u'type': u'string'},
                                                      u'additionalItems': {u'anyOf': [{u'type': u'boolean'},
                                                                                      {u'$ref': u'#'}],
                                                                           u'default': {}},
                                                      u'additionalProperties': {u'anyOf': [{u'type': u'boolean'},
                                                                                           {u'$ref': u'#'}],
                                                                                u'default': {}},
                                                      u'allOf': {u'$ref': u'#/definitions/schemaArray'},
                                                      u'anyOf': {u'$ref': u'#/definitions/schemaArray'},
                                                      u'default': {},
                                                      u'definitions': {u'additionalProperties': {u'$ref': u'#'},
                                                                       u'default': {},
                                                                       u'type': u'object'},
                                                      u'dependencies': {u'additionalProperties': {u'anyOf': [{u'$ref': u'#'},
                                                                                                             {u'$ref': u'#/definitions/stringArray'}]},
                                                                        u'type': u'object'},
                                                      u'description': {u'type': u'string'},
                                                      u'enum': {u'minItems': 1,
                                                                u'type': u'array',
                                                                u'uniqueItems': True},
                                                      u'exclusiveMaximum': {u'default': False,
                                                                            u'type': u'boolean'},
                                                      u'exclusiveMinimum': {u'default': False,
                                                                            u'type': u'boolean'},
                                                      u'id': {u'format': u'uri',
                                                              u'type': u'string'},
                                                      u'immutable': {u'default': False,
                                                                     u'type': u'boolean'},
                                                      u'items': {u'anyOf': [{u'$ref': u'#'},
                                                                            {u'$ref': u'#/definitions/schemaArray'}],
                                                                 u'default': {}},
                                                      u'maxItems': {u'$ref': u'#/definitions/positiveInteger'},
                                                      u'maxLength': {u'$ref': u'#/definitions/positiveInteger'},
                                                      u'maxProperties': {u'$ref': u'#/definitions/positiveInteger'},
                                                      u'maximum': {u'type': u'number'},
                                                      u'minItems': {u'$ref': u'#/definitions/positiveIntegerDefault0'},
                                                      u'minLength': {u'$ref': u'#/definitions/positiveIntegerDefault0'},
                                                      u'minProperties': {u'$ref': u'#/definitions/positiveIntegerDefault0'},
                                                      u'minimum': {u'type': u'number'},
                                                      u'multipleOf': {u'exclusiveMinimum': True,
                                                                      u'minimum': 0,
                                                                      u'type': u'number'},
                                                      u'not': {u'$ref': u'#'},
                                                      u'oneOf': {u'$ref': u'#/definitions/schemaArray'},
                                                      u'pattern': {u'format': u'regex',
                                                                   u'type': u'string'},
                                                      u'patternProperties': {u'additionalProperties': {u'$ref': u'#'},
                                                                             u'default': {},
                                                                             u'type': u'object'},
                                                      u'position': {u'minimum': 0,
                                                                    u'type': u'number'},
                                                      u'properties': {u'additionalProperties': {u'$ref': u'#'},
                                                                      u'default': {},
                                                                      u'type': u'object'},
                                                      u'required': {u'default': False,
                                                                    u'type': u'boolean'},
                                                      u'title': {u'type': u'string'},
                                                      u'type': {u'anyOf': [{u'$ref': u'#/definitions/simpleTypes'},
                                                                           {u'items': {u'$ref': u'#/definitions/simpleTypes'},
                                                                            u'minItems': 1,
                                                                            u'type': u'array',
                                                                            u'uniqueItems': True}]},
                                                      u'uniqueItems': {u'default': False,
                                                                       u'type': u'boolean'}},
                                      u'type': u'object'}},
     'type': 'object'}

On instance['parameters']:
    None
2015-01-30 07:57:31,171 DEBUG [-] Loading action from /opt/stackstorm/packs/rackspace/actions/list_vm_sizes.yaml.