Add snappy variant
    
    As part of ongoing work to build a Fedora base snap under
    the auspices of the Server WG, we need a specific fedora-release
    variant to identify the base snap similar to how the Fedora base
    container is identified.
    
    This change provides that in the form of the 'fedora-release-snappy'
    subpackage.
    
    Signed-off-by: Neal Gompa <ngompa13@gmail.com>