roles/nginx: Add "ansible managed" string to configs

Generates a placeholder text to say that the file is managed by
ansible.

roles/nginx/templates/vhost.conf.j2

@@ -1,3 +1,5 @@
+# {{ ansible_managed }}
+
 {# helper variables and per-site defaults that we can't set in role defaults #}
 {% set domain_name    = item.domain_name %}
 {% set domain_aliases = item.domain_aliases | default("") %}