Add ensure parameter to zuul services

Set it initially as undef, so it's not affecting service
behaviour. But it will allow the possibility to zuul
consumers to set it as running if needed.

Change-Id: If77f187ec1f404bb2d13ab99ea937b5bd31e08f3

manifests/merger.pp

@@ -16,9 +16,11 @@
 # == Class: zuul::merger
 #
 class zuul::merger (
+  $ensure = undef,
   $manage_log_conf = true,
 ) {
   service { 'zuul-merger':
+    ensure     => $ensure,
     name       => 'zuul-merger',
     enable     => true,
     hasrestart => true,

manifests/server.pp

@@ -16,10 +16,12 @@
 # == Class: zuul::server
 #
 class zuul::server (
+  $ensure = undef,
   $layout_dir = '',
   $manage_log_conf = true,
 ) {
   service { 'zuul':
+    ensure     => $ensure,
     name       => 'zuul',
     enable     => true,
     hasrestart => true,