Add example lines to OpenDKIM TrustedHosts file. Fixes #16.

roles/mailserver/templates/etc_opendkim_TrustedHosts.j2

 127.0.0.1
-# TODO add your server's IP and DNS hosts
+# TODO add your server's IP and DNS hosts, ex:
+# ns1.linode.com
+# 198.58.112.239 
 {% for domain in mail_virtual_domains %}
 {{ domain.name }}
 {% endfor %}