instance_group_member records not purged from nova_api DB for deleted instances

[Matt Riedemann]

Instance group member records used to be in the cell databases but were moved to the API database in Ocata. Previously, when deleting an instance in the cell, we'd also delete it's instance group membership record in the same cell database. Now that instance group membership is modeled in the API DB, we don't cleanup the instance group membership record when an instance is deleted.

The server groups API handles this by filtering out deleted instances which are still listed as group members:

https://github.com/openstack/nova/blob/351faa959fc228c726848a68e6ea3f3dadde025e/nova/api/openstack/compute/server_groups.py#L47

However, we could make that more efficient if we didn't have to do those queries / filtering in the first place, by removing the nova_api.instance_group_member records when archiving the database, like we do for instance_mappings request_specs records:

https://github.com/openstack/nova/blob/351faa959fc228c726848a68e6ea3f3dadde025e/nova/cmd/manage.py#L553-L561

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/588943