zuul: split conf file for server and merger

We can get Zuul merger process on different boxes.  The zuul.conf has
the Jenkins API key and I only want it to be exposed on the Zuul server.

This generates a zuul-merger.conf file that does not have the [jenkins]
section. Merger only requires the gearman, gerrit, merger, and zuul
sections per upstream documentation.

Make the merger to subscribe to that configuration file.  We do not
notify since we might not have a merger process (installed via
zuul::merger) on the node.

Change-Id: I1cbfa8446311b18c9075da023051b528548ae017