Notification support for host aggregate related operation

Add notification for the following operations related to host
aggregation:
1) Send notification when create a new host aggregation
2) Send notification when delete a host aggregation
3) Send notification when add host to a host aggregation
4) Send notification when delete host from host aggregation
5) Send notification when update host aggregation for aggregate
name and available zone
6) Send notification when update host aggregation meta data

Change-Id: I2dc42addb587c4765b41e464697f35b1f844fde9
Implements: blueprint notification-host-aggregate
Merge "Notification support for host aggregate related operation"