consumer: allow configuration of arches for update tests
    
    This adds a config setting for the job scheduling consumer which
    allows configuration of the arches for which update jobs will be
    scheduled. This is to allow us to enable testing of updates on
    ppc64le in Fedora staging as a test; if that goes well, we will
    enable them in production, and also later enable update tests on
    aarch64 once we have more workers.
    
    The new setting is required, so if anyone else is actually using
    these consumers, you'll need to update your configuration.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>