Implemented Interface entity with all its fields and basic operations

[abalakh]

• Implemented Interface entity (as before it was basically a stub with
  no available operations and even wrong path)
• Added all possible fields variations
• Implemented unit tests

Key features:

• Interface requires a host id in its path, so some additional logic
  was required for initialization, read() and search()
• Set of available Interface fields depends on Interface's type
  (some fields are only returned for specific type), extra logic was
  requred here too

[abalakh]

Test results:

>>> host = entities.Host().create()
>>> host.interface
[nailgun.entities.Interface(nailgun.config.ServerConfig(url='https://example.com', verify=False, auth=('admin', 'password')), host=nailgun.entities.Host(nailgun.config.ServerConfig(url='https://example.com', verify=False, auth=('admin', 'password')), id=3), id=8)]
>>> host.interface[0].read()
nailgun.entities.Interface(nailgun.config.ServerConfig(url='https://example.com', verify=False, auth=('admin', 'password')), domain=nailgun.entities.Domain(nailgun.config.ServerConfig(url='https://example.com', verify=False, auth=('admin', 'password')), id=3), managed=True, ip=None, primary=True, mac=u'7a:95:88:94:3e:9c', provision=True, id=8, name=u'ynygrucya.ejfsnanrgc', subnet=None, host=nailgun.entities.Host(nailgun.config.ServerConfig(url='https://example.com', verify=False, auth=('admin', 'password')), id=3), virtual=False, identifier=None, type=u'interface')
>>> interface = entities.Interface(id=8, host=3).read()
>>> interface.name = 'newnameforinterface'
>>> interface.update(['name'])
nailgun.entities.Interface(nailgun.config.ServerConfig(url='https://example.com', verify=False, auth=('admin', 'password')), domain=nailgun.entities.Domain(nailgun.config.ServerConfig(url='https://example.com', verify=False, auth=('admin', 'password')), id=3), managed=True, ip=None, primary=True, mac=u'7a:95:88:94:3e:9c', provision=True, id=8, name=u'newnameforinterface.ejfsnanrgc', subnet=None, host=nailgun.entities.Host(nailgun.config.ServerConfig(url='https://example.com', verify=False, auth=('admin', 'password')), id=3), virtual=False, identifier=None, type=u'interface')
>>> entities.Interface(host=3, id=8).search()
[nailgun.entities.Interface(nailgun.config.ServerConfig(url='https://example.com', verify=False, auth=('admin', 'password')), domain=nailgun.entities.Domain(nailgun.config.ServerConfig(url='https://example.com', verify=False, auth=('admin', 'password')), id=3), managed=True, ip=None, primary=True, mac=u'7a:95:88:94:3e:9c', provision=True, id=8, name=u'newnameforinterface.ejfsnanrgc', subnet=None, host=nailgun.entities.Host(nailgun.config.ServerConfig(url='https://example.com', verify=False, auth=('admin', 'password')), id=3), virtual=False, identifier=None, type=u'interface')]
>>> entities.Interface(host=3).create()
nailgun.entities.Interface(nailgun.config.ServerConfig(url='https://example.com', verify=False, auth=('admin', 'password')), domain=None, managed=True, ip=u'15.2.12.168', primary=False, mac=u'50:c8:ad:79:7d:cf', provision=False, id=16, name=u'yjhpiyubg', subnet=None, host=nailgun.entities.Host(nailgun.config.ServerConfig(url='https://example.com', verify=False, auth=('admin', 'password')), id=3), virtual=False, identifier=None, type=u'interface')

[coveralls]

Coverage increased (+0.02%) to 98.605% when pulling 172fb56 on abalakh:host_interfaces into 602a4a5 on SatelliteQE:master.

[ldjebran]

ACK

[svtkachenko]

ACK