Tweak and improve generic compose 'version' implementation
    
    This slightly messy split, AFAIR, more or less just strips off
    the dist - except it doesn't work for updates and updates-testing
    composes, as they have '-updates-' or '-updates-testing' in the
    middle of their compose IDs. So let's do this in a way that's
    simpler and should work better.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>