Add notification for server group operations

Currently, there is no notification when create/update/delete
server groups, this caused some 3rd party client cannot know
when those operations are finished.

The fix was adding notifications for those operations.

DocImpact
The wiki page https://wiki.openstack.org/wiki/SystemUsageData
needs to be updated once this patch was merged.

Change-Id: Ia1f4a6752668ddb73ebb8faf525b58a28d183754
Closes-Bug: #1343200