Add notifications for identity CRUD events

This patch adds support for notifications for the following
resources:

  user, group, project, role, trust

on the following events:

  create, delete, update

For more information on what events keystone supports visit:
http://docs.openstack.org/developer/keystone/event_notifications.html

Change-Id: I4ff07803537daf6937fba02e9ae8fe2afec938e5
Merge "Add notifications for identity CRUD events"