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