Refresh nailgun systemd units if settings.yaml is updated

This was caused by starting systemd services before applying
Puppet, which prevented Puppet changes to settings from taking
effect immediately.

Change-Id: I9f644b829601b68182b397989e43bf4f80773975
Closes-Bug: #1526874

deployment/puppet/nailgun/examples/nailgun-only.pp

@@ -183,6 +183,10 @@ if $use_systemd {
     services   => $services,
     require    => Class['nailgun::venv']
   }
+  if ($production == 'prod') or ($production == 'docker') {
+    File['/etc/nailgun/settings.yaml'] ~> Service[$services]
+  }
+
 } else {
   class { 'nailgun::supervisor':
     service_enabled => false,

tests/noop/spec/hosts/master/nailgun-only_spec.rb

@@ -92,6 +92,7 @@ describe manifest do
             :ensure    => 'running',
             :enable    => 'true',
           })
+          should contain_file('/etc/nailgun/settings.yaml').that_notifies("Service[#{service}]")
         end
       end
     end # context