image_build: fix subvariant handling
    
    Dennis added a 'subvariant' property to the Docker image config,
    but we weren't using it, we were just hard-coding the variant
    as the subvariant...and doing it wrong, as well, using the
    variant object rather than its uid property.
    
    This fixes both problems: we'll now use the explicitly set
    image config 'subvariant' property if there is one, otherwise
    we use the variant.uid as the subvariant.
    
    Note this is a high priority fix, as it broke F24 compose today.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>