Use httpd::vhost $content parameter

Use the new $content parameter of httpd::vhost instead of $template.
This way, the template gets rendered within the scope where it assumes
its variables are, and doesn't need to use the scope object which
doesn't work within a defined type. This will ensure the template keeps
working on puppet 4.

Depends-On: https://review.openstack.org/570824
Change-Id: Iaa31189ace6140327af034fd8209211077d7c552

manifests/site.pp

@@ -102,7 +102,7 @@ define mailman::site (
     port     => 80,
     docroot  => '/var/www/',
     priority => '50',
-    template => 'mailman/mailman_multihost.vhost.erb',
+    content  => template('mailman/mailman_multihost.vhost.erb'),
   }
 
   file { "/etc/init.d/mailman-${name}":