Comment 4 for bug 1809165

Reviewed: https://review.openstack.org/626578
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=3b18ab4b32f7720c961a88865b781e77ca731501
Submitter: Zuul
Branch: stable/rocky

commit 3b18ab4b32f7720c961a88865b781e77ca731501
Author: Matt Riedemann <email address hidden>
Date: Wed Dec 19 15:31:16 2018 -0500

    Document using service user tokens for long running operations

    A recent thread in the mailing list [1] reminded me that we
    don't have any documentation for the service user token feature
    added back in ocata under blueprint use-service-tokens.

    This change adds a troubleshooting entry for when using service
    user tokens would be useful, and links to it from two known
    trouble spots: live migration timeouts and creating images.

    [1] http://lists.openstack.org/pipermail/openstack-discuss/2018-December/001130.html

    Change-Id: I1dda889038ffe67d53ceb35049aa1f2a9da39ae8
    Closes-Bug: #1809165
    (cherry picked from commit 1a4cc6c96dc9acd5e6bc467f85cb77dd9e32c8d4)