[F27] Disable cups.service
FESCo[1] and the Server SIG[2] decided to switch to using the
socket-activated CUPS service rather than have it running at all
times.
[2] https://pagure.io/fesco/issue/1701
[2] https://pagure.io/fedora-server/issue/1
Signed-off-by: Stephen Gallagher <sgallagh@redhat.com>