release-candidate.sh: fix release version / label handling
    
    A compose 'label', as defined by productmd, does not contain the
    release version. It has to be something like "Beta-1.3" or
    "RC-3.1".
    
    Instead of having the script require a string that isn't a label,
    but calling it a label and passing it as one to pungi-fedora
    (which rejects it), have the script require separate arguments
    for the release number and the label, and use them appropriately.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>