zuul: get rid of git_dir and zuul_url in server conf

Both paramaeters are now being used by the merger process and already in
[merger] section.

Change-Id: I3b034976a549956bacd2c4e2f5cd9c27cfa30b02
zuul: migrate merger definitions to merger.pp

Move around zuul-merger related puppet definitions out of init.pp in
favor of merger.pp.

Change-Id: I87c86ed791e5d8dc94c4b47a4305eba34cc69558
zuul: migrate server definitions to server.pp

Move around zuul server related puppet definitions out of init.pp in
favor of server.pp. Still have to migrate the zuul.conf template but it
is shared with merger.

Change-Id: Ied2ed92dfbc8326f8a9b910c1ac7ffdab78e48d0
Merge changes Ied2ed92d,I87c86ed7,I3b034976 into production

* changes:
  zuul: migrate server definitions to server.pp
  zuul: migrate merger definitions to merger.pp
  zuul: get rid of git_dir and zuul_url in server conf