Add Zuul merge servers

Remove obsolete config options from zuul module.

The server and merger classes are constructed so that they
may coexist on a single server.  Also, the init section is
constructed so that it will install everything needed for
both services but without activating them (leaving maximum
flexibility for the operator).

Change-Id: I7b86fbbe4611c5edfb463a0a6944e0717f664188
Merge "Add Zuul merge servers"