Tweak ostree build and rebase for update tests to be more robust
    
    We have had several problems where rebasing from one release to
    another just doesn't work, and we have to work around it somehow.
    Sometimes it's difficult or impossible to do. All we want to do
    here is check the rebase mechanism itself; we don't actually want
    to assert that you can rebase to any specific other release.
    
    For update tests, we should be able to use a non-standard ref
    name for the ostree we build, embed into the installer image,
    and install. That should mean we can then rebase to the standard
    ref name for the same release, which should be much safer than
    trying to rebase to a different release. We can't do this for
    the compose tests, but at least for update tests I'm hoping this
    makes things more robust.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>