Comment 0 for bug 1639152

When create instance with server group information provided, nova will send out notification
about "servergroup.addmember":
https://git.openstack.org/cgit/openstack/nova/tree/nova/objects/instance_group.py#n425

Services like Searchlight and Telemetry are listen to the notification to do their jobs
Take Searchlight as an example, when an instance has been added to a server group SL will
catch the notification and index the up-to-date data to its database for searching perpose.

Currently, when delete instances, the related server group's member field will also be
cleaned, but no notification will be send out (nova performs direct DB action). Take
again SearchLight as an example, it cannot perform sync actions under this situation,
thus the indexed data will be out-of-date.

We should send out notification about server group changes when delete instances.