Add notifications to Sahara

Added following notifications to Sahara:
 * Notification about creating cluster
 * Notification about deleting cluster
 * Notification about updating cluster status

Added unit tests for messaging and notify functions
Docs updated

Partially implement: blueprint ceilometer-integration

Change-Id: I7add0e477ab16f189c533857ad6a158ee41242de
Merge "Add notifications to Sahara"