Bump minimal image size again (but stealth this time)
    
    We need minimal to be 18GB now, because we're booting from it for
    the Workstation live image build test. Not doing a version bump
    as they turn out to be a bit of a pain to deal with, I should
    enhance that somehow.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>