Add forwarder plugin for federated Alerta

[satterly]

Add alert, action and delete forwarder plugin for federated Alerta environments

An example config for 2 x MoM (Manager-of-Managers) and 2 x MLMs (Mid-Level Managers) can be found in examples/plugins/forwarder

Minimal server config ...

BASE_URL='http://mlm1.local.alerta.io:10001'   # must match actual server name and port
PLUGINS=['forwarder']

Example forwarding config...

FWD_DESTINATIONS = [
    ('http://localhost:9000', {'username': 'user', 'password': 'pa55w0rd', 'timeout': 10}, ['alerts', 'actions']),  # BasicAuth
    # ('https://httpbin.org/anything', dict(username='foo', password='bar', ssl_verify=False), ['*']),
    ('http://localhost:9001', {
        'key': 'e3b8afc0-db18-4c51-865d-b95322742c5e',
        'secret': 'MDhjZGMyYTRkY2YyNjk1MTEyMWFlNmM3Y2UxZDU1ZjIK'
    }, ['actions']),  # Hawk HMAC
    ('http://localhost:9002', {'key': 'demo-key'}, ['delete']),  # API key
    ('http://localhost:9003', {'token': 'bearer-token'}, ['*']),  # Bearer token
]

Note: It is recommended to use HMAC Auth for federated Alerta as the credentials don't need to be associated with any users, and do not expire.

To generate HMAC key and secret, it is useful to use UUID for key and base64 encoded string for secret so that they are visibly different.

On macOs, run:

$ uuidgen | tr '[:upper:]' '[:lower:]'         <= create HMAC "key"
58e7c66f-b990-4610-9496-60eb3c63339b
$ date | md5 | base64                        <= create HMAC "secret"
MzVlMzQ5NWYzYWE2YTgxYTUyYmIyNDY0ZWE2ZWJlYTMK

Fixes #1147 #1153

[coveralls]

Coverage decreased (-1.06%) to 73.01% when pulling f893b2c on alert-forwarder-plugin into 1c23c6a on master.