Handle rename of AtomicWorkstation to Silverblue
    
    We updated the dispatcher code, but not the templates...so we
    didn't have any actual tests to run on Silverblue images. Let's
    fix that. Note this means we won't test F28 and earlier images
    that still have 'AtomicWorkstation' as their subvariant any
    more, unless I set up some sorta workaround in the dispatcher.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>