Disable 'updates' repo for Branched update tests
    
    This works around RHBZ #1552814, and it's not incorrect really
    because the repo is always empty for Branched. I didn't do it
    before because we might theoretically start using the repo for
    Branched at some point in the future, and if we did that we'd
    probably want it enabled for this test. But to get F28 update
    tests working, let's just turn it off for now.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>