Fix up the 'skip update on support server' stuff (a20ea59)
    
    The previous attempt at this (in a20ea59) was wrong, because it
    meant we didn't create the update repo at all, and we do need
    the repo to exist in the support_server test every time (as the
    point of the support server is to serve it out via NFS). We just
    don't want to *use it on the support_server system itself* if
    the releases don't match. So rejig things to do that properly.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>