Add comment filter to {{ansible_managed}} string

Multiline {{ansible_managed}} strings do not get properly commented
without the comment filter.
See http://docs.ansible.com/ansible/playbooks_filters.html#comment-filter

templates/hardening.conf.j2

@@ -1,4 +1,4 @@
-# {{ansible_managed}}
+# {{ansible_managed|comment}}
 # Additional configuration for Nginx.
 
 client_header_buffer_size {{nginx_client_header_buffer_size}};