rgmanager: Add 'disabled' configuration option
    
    The option turns off rgmanager, which will shut down if
    running and cause rgmanager to refuse to start if
    starting up.
    
    Resolves: rhbz#723925
    
    Signed-off-by: Lon Hohberger <lhh@redhat.com>
    Reviewed-by: Fabio M. Di Nitto <fdinitto@redhat.com>