Provide way to specify repo ref

We don't always want to use just 'master' since that may result in races
when running puppet across many nodes while project_config is changing.
Allow for a way to specify the ref we want via facter with a fallback to
master if the fact does not exist.

Change-Id: If1c17e55c7f1b15d28748761c3ee99af88fada40

manifests/init.pp

@@ -22,11 +22,19 @@ class project_config(
 
   $config_dir = Vcsrepo['/etc/project-config']
 
+  # Note project_config_ref is expected to be provided by facter
+  # if you want to use it.
+  if ($::project_config_ref != undef) {
+    $revision = $::project_config_ref
+  } else {
+    $revision = 'master'
+  }
+
   if (!defined($config_dir)) {
     vcsrepo { '/etc/project-config':
       ensure   => latest,
       provider => git,
-      revision => 'master',
+      revision => $revision,
       source   => $url,
     }
   }