Set a custom bundle name format for GCE images
    
    KIWI currently has an issue where using the custom format
    results in the wrong file extension being generated for the image.
    
    This causes Koji to fail the image for Pungi to gather it.
    
    Work around it by adding the ".tar" extension manually.
    
    Reference: https://github.com/OSInside/kiwi/issues/2628
    
    Signed-off-by: Neal Gompa <ngompa@fedoraproject.org>