Report a test priority to ResultsDB
    
    See https://phab.qa.fedoraproject.org/T902 for more on this.
    We decided for the short term, to enable releng to make
    decisions about compose quality, to have test systems forward
    a test priority value as ResultsDB extradata, at least until
    there's a better system in place for doing this. A value of
    '2' for 'priority.fedora.compose' means 'this test must pass
    for a Fedora compose to be considered of Alpha quality'. A
    value of '5' maps to an 'Optional' test case, i.e. it's really
    unimportant and won't be used for any kind of gating. I will
    probably add the value and some explanation of the meanings to
    resultsdb_conventions soon.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>