Handle scheduling update live image create and install tests
    
    This also rethinks the flavor naming strategy a bit, to use
    names that are more specific to the image being built and
    more similar to the flavors used for compose tests. I tried
    using the *actual same* flavors the compose tests use, but
    that leads to jobs being scheduled for updates as if they
    were composes; there's a feature you can use to prevent that,
    but it just seemed like it was getting a bit complicated and
    I preferred this approach in the end, for now.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>