Add new greenwave policy for critical path updates
    
    This adds a new Greenwave policy requiring all openQA update
    tests to pass for an update to be pushed to testing or stable.
    It's intended to be queried by Bodhi only for critical path
    updates (as openQA does not test non-critpath updates ATM).
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>