Send out notification about server group changes when delete instances

[Zhenyu Zheng]

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):
https://git.openstack.org/cgit/openstack/nova/tree/nova/db/sqlalchemy/api.py#n1862
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thus the indexed data will be out-of-date.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.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/393649

[OpenStack Infra (hudson-openstack)]

Change abandoned by Zhenyu Zheng (<email address hidden>) on branch: master
Review: https://review.openstack.org/393649

[Balazs Gibizer (balazs-gibizer)]

I don't think this is in progress any more as the patch is abandoned

[Balazs Gibizer (balazs-gibizer)]

It was discussed in the Rocky PTG and agreed to do this. A new specless bp has been created to track the effort https://blueprints.launchpad.net/nova/+spec/add-server-group-remove-member-notifications