zuul: allow gearman access from mergers

We will get Zuul merger installed on lanthanum in addition to gallium.
Manually maintain a list of such hosts in network::constant and adjust
the ferm rule to allow them to reach gearman.

Note: the ferm::rule is being renamed.

Change-Id: Ie27fd16d3f1a0b0a6d8b625cd8d5dfea971c8212
Merge "zuul: allow gearman access from mergers" into production