Comment 14 for bug 1712008

Reviewed: https://review.openstack.org/496729
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=b37411c1e3d167156b6e54f837fc291c9e61eeef
Submitter: Jenkins
Branch: stable/pike

commit b37411c1e3d167156b6e54f837fc291c9e61eeef
Author: Matt Riedemann <email address hidden>
Date: Tue Aug 22 16:54:44 2017 -0400

    Restrict live migration to same cell

    We do not yet support live migrating an instance
    across cells. This change handles two cases for
    live migration:

    1. The destination host is forced so the scheduler
       is bypassed. In this case we directly compare the
       source cell against the destination cell and fail
       if they are not the same with a MigrationPreCheckError.

    2. If no destination host is specified, or it's not forced,
       we update the RequestSpec sent to the scheduler so it
       will restrict the compute nodes it pulls from the same
       cell that the instance lives in. If a host is requested
       in this case but it's in a different cell, it would result
       in a NoValidHost error from the scheduler.

    Change-Id: I66fc72d402ac118270a835cf929fe1ea387d78cd
    Closes-Bug: #1712008
    (cherry picked from commit 8fc789deb7c774bcc5b5128d638c6c7e30bf0a54)