Implements notification listener and dispatcher

This patch allows to quickly create a listener to receive
notification messages.

Example of the api:

class Endpoint(object):
    def warn(self, ctxt, publisher_id, event_type, payload):
        do_something(payload)

target = messaging.Target(topic='notifications', exchange='cinder')
listener = notify.get_notification_listener(transport, [target],
                                            [Endpoint()],
                                            executor,
                                            serializer)

Implements blueprint notification-subscriber-server

Change-Id: I434bc487c382a2048670df726d9bebd640150bb9
Merge "Implements notification listener and dispatcher"