openqa/dispatcher: use arch filtering instead of custom WANTED
    
    I just enhanced the scheduler code so we can share the stock
    WANTED definition (which now includes all arches) between prod
    and lab, but filter the arches with a config file setting. This
    means we don't have to carry and install a whole custom WANTED
    file with the extra arches for lab any more, we just set the
    appropriate value in the config file. Also drop some stuff from
    the config file that's been useless since we switched to
    fedora-messaging.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>