zuul: migrate settings to role::zuul::configuration

Further rely on role::zuul::configuration. Will soon let us merge both
realm based classes and then split them by role (server/merger).

I also changed the Gearman server IP from 127.0.0.1 to the public
address of gallium.wikimedia.org.  The Zuul merger will need to be able
to reach it so I widened the ferm rule that protects the Gearman port.
We will later on need to collect any server having zuul::merger applied.

Change-Id: I4cff5cfe27401af7c2d2f21496dcc426cc197b51