zuul: install zuul from role classes

Part of phasing out zuulwikimedia class. The ::zuul class invocation is
now done directly from the role.

Also renamed $git_branch to $git_source_branch and added very basic
documentation of the parameters for the ::zuul class.

Change-Id: I060de5a3dc699d0fb5c583908ee36dd2d0fe3cd3
zuul: move Icinga checks to zuul::monitoring::server

Part of phasing out zuulwikimedia, move nrpe definitions to a new
zuul::monitoring::server class.  Will later add one for merger.

The reason is that some servers will only run the merger, so we will not
want to monitor the server on those nodes.

Change-Id: I99bfb7a4a01d29925443fb4424b4341f26fe0e1b
zuul: monitor Zuul merger via nrpe

Zuul has split the scheduler and merger operations in two daemon. The
new one (zuul-merger) remained unmonitored which is unfortunate.

We will have nodes having only a merger daemon, so create a new class
zuul::monitoring::merger

I verified the command manually on gallium which has a merger running:

gallium:~$ /usr/lib/nagios/plugins/check_procs -w 1:1 -c 1:1 \
--ereg-argument-array '^/usr/bin/python /usr/local/bin/zuul-merger'
PROCS OK: 1 process with regex args '^/usr/bin/python
/usr/local/bin/zuul-merger'
gallium:~$

The inclusion is still done in zuulwikimedia for sake of clarity. Will
be moved around later on.

Change-Id: I493494e0a545496bd4036ba350a9e748ea535f9d
Merge changes I493494e0,I99bfb7a4,I060de5a3 into production

* changes:
  zuul: monitor Zuul merger via nrpe
  zuul: move Icinga checks to zuul::monitoring::server
  zuul: install zuul from role classes