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
Abstract the acknowledge layer of a message

The patch add ta abstraction layer to acknowledge a message.

Partial implements blueprint notification-subscriber-server

Change-Id: I6e37780cc28737cfd56b6719ec8d9cebbc9bb278
Merge "Implements notification listener and dispatcher"
Merge "Abstract the acknowledge layer of a message"
Don't reply to notification message

The notification listener doesn't have anything to send to the notifier
and the notifier doesn't attend to receive something.

So this patch remove the message reply when the listener is a
notification
listener.

Partial implements blueprint notification-subscriber-server

Change-Id: Ic989947ba3b6894cde788422842fca19159ea261
Make the dispatcher responsible of the message ack

This patch make the dispatcher responsible of the message
acknowledgement.

This is the preliminar step to be able to requeue message.

Partial implements blueprint notification-subscriber-server

Change-Id: If74b47d5e23976d407deb27df7395b1982963c75