Notification for network/subnet/port create/delete/update.

Notification for network/subnet/port create/delete/update.
blueprint quantum-notifications

Add host configuration into conf, which is determined by socket.gethostname()
function by default. Host name is part of publiser id, which is in the format
of 'network'.$host.

We add create/update/delete start and end notification for each kind of resource.
By default, the notification do nothing since the notifier driver is no_op_notifier.
We can define it in quantum.conf.

Change-Id: Ibc5eacac7a324584e6ccff120f573444932a88ef