zuul: phase out zuulwikimedia

Replace realm based roles by ... role based roles!  This introduces:

 role::zuul::merger
 role::zuul::server

That makes zuulwikimedia::instance useless. The previous roles
(labs/production) are kept around for backward compatibility.

Change-Id: Ib05cdd7dda96b29f34086b8fe071f89726d19110