add 'subvariant' image property, create live/appliance names
    
    This adds the new 'subvariant' property to all images produced by Pungi.
    For most image types this is the same as the variant, for now. For
    live_images and livemedia_phase, the image config may specify
    'subvariant' and the value will be used if present, otherwise the
    variant name will be used.
    
    We also now synthesize the Koji image 'name' for live and appliance
    images if the image config doesn't specify it, and use a name that
    complies better with the naming policy for ARM disk images (partially
    resolving https://pagure.io/pungi-fedora/issue/7 ).
    
    This is completely untested as yet, just bashing stuff out on a Sunday,
    will work more on it tomorrow.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>