Propose: Add the notification support to the backup service

The volume backup service needs to send the notifications to ceilometer
to report the usage stats for the volume backup during backup create,
delete and restore. In addition, progress notification support will be
added for some backup drivers, which can send periodic notifications
to Ceilometer.

Related to blueprint backup-notification

Change-Id: Ic55403ba83f8ec6b6c7a6a77135d4687c71659bb
Merge "Propose: Add the notification support to the backup service"