zuul: learn the ability to set push_change_refs

push_change_refs controls whether zuul should push its reference back in
Gerrit.  This might not be always wanted, for example on very busy
Gerrit installation that never use the refs/zuul/* hierarchy for any
purpose.

Pushing back to Gerrit is now disabled by default and is explicitly
enabled on OpenStack infrastructure (openstack_project::zuul).

Change-Id: I24111ba4b20ab3d5198fdf7deaf902485260d8a5
Reviewed-on: https://review.openstack.org/16475
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Tested-by: Jenkins

manifests/init.pp

@@ -7,7 +7,8 @@ class zuul (
   $gerrit_user,
   $url_pattern,
   $status_url = "https://${::fqdn}/zuul/status",
-  $git_source_repo = 'https://github.com/openstack-ci/zuul.git'
+  $git_source_repo = 'https://github.com/openstack-ci/zuul.git',
+  $push_change_refs = false
 ) {
   $packages = [
     'python-webob',

templates/zuul.conf.erb

@@ -13,6 +13,6 @@ layout_config=/etc/zuul/layout.yaml
 log_config=/etc/zuul/logging.conf
 state_dir=/var/lib/zuul
 git_dir=/var/lib/zuul/git
-push_change_refs=true
+push_change_refs=<%= push_change_refs %>
 url_pattern=<%= url_pattern %>
 status_url=<%= status_url %>