Add notification for host operation

Add notification for the following host operations:
1) compute/api.py:set_host_enabled
2) compute/api.py:host_power_action
3) compute/api.py:set_host_maintenance

This can make sure 3rd party callers can get host status quickly.

Implements bp host-operation-notification

Change-Id: Ib4cc80e568b6028afbdd6cb86f6ac8edeed4f933
Merge "Add notification for host operation"