bodhi/backend: clean and fix the owner-sync-pagure script
    
    Turns out we still carried i586 arch in there while it's long gone
    in koji.
    
    The list of extra arches needs to passed as a string, not a list
    and a space delimited string, not coma. Otherwise koji crashes on
    the other end, but this way it seems to work.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>