Comment 7 for bug 1790221

Reviewed: https://review.openstack.org/606086
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=b3060a9d3103fb8ade6ebeaa53cb4dd39308f8af
Submitter: Zuul
Branch: stable/rocky

commit b3060a9d3103fb8ade6ebeaa53cb4dd39308f8af
Author: Matt Riedemann <email address hidden>
Date: Fri Aug 31 18:14:25 2018 -0400

    Null out instance.availability_zone on shelve offload

    When a user shelve offloads a server, the compute manager
    nulls out the instance host and node attributes. However
    the availability_zone attribute is left set on the instance
    so the API will show the instance as in an AZ when really
    it's not, because an instance that is not on a host is not
    in a host aggregate so it can't be in an AZ.

    Keep in mind that there are two ways an instance can be in
    an AZ:

    1. The user specifically requests to create the server in
       the AZ.

    2. The user does not request an AZ and one is assigned via
       the selected host during server create (or resize, etc).

    For the first case, the server will always remain in the
    user-requested AZ even after shelve/unshelve. But in the
    second case, unshelving the server can result in the server
    being spawned on a new host in a different AZ - the scheduler
    does not restrict the AZ in that second case.

    This change nulls out the instance.availability_zone just like
    the host and node fields on shelve offload since it's
    confusing to show a shelved offloaded server in an AZ that
    doesn't have a host.

    Final note: the _nil_out_instance_obj_host_and_node method
    is also called during server create if the build fails and
    is aborted or rescheduled to another host, and in the case
    that unshelve fails. In the case of a server build reschedule,
    conductor will set the instance.availability_zone appropriately
    based on the alternate host for the reschedule. In the other
    failure cases, leaving the instance AZ null is appropriate.

    Conflicts:
          doc/notification_samples/instance-delete-end_not_scheduled.json
          doc/notification_samples/instance-delete-start_not_scheduled.json

    NOTE(mriedem): The conflict is due to not having change
    If0693eab2ed31b5fbfe6cbafa5d67b69c2ed8442 in Rocky.

    Change-Id: I25a4f36027390def83cfe25f4f3b4af9660da502
    Closes-Bug: #1790221
    (cherry picked from commit 771b9eaa71742b7a158c2e7759a3046ea5a6fc3a)