Comment 4 for bug 1732947

Reviewed: https://review.openstack.org/521391
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=54407afef3b446e34c44ba5392ccbf121e266de2
Submitter: Zuul
Branch: master

commit 54407afef3b446e34c44ba5392ccbf121e266de2
Author: Matt Riedemann <email address hidden>
Date: Sun Nov 19 17:25:56 2017 -0500

    Get original image_id from volume for volume-backed instance rebuild

    A volume-backed instance will not have the instance.image_ref
    attribute set, to indicate to the API user that it is a volume-backed
    instance.

    Commit 984dd8ad6add4523d93c7ce5a666a32233e02e34 missed this subtle
    difference with how instance.image_ref is used, which means a rebuild
    of any volume-backed instance will now run through the scheduler, even
    if the image_href passed to rebuild is the same image ID as for the
    root volume.

    This fixes that case in rebuild by getting the image metadata off the
    root volume for a volume-backed instance and compares that to the
    image_href passed to rebuild.

    Change-Id: I48cda813b9effa37f6c3e0cd2e8a22bb78c79d72
    Closes-Bug: #1732947