[Reviewer: Andy] Basic IPv6 support

Not rigorously tested in all cases, but I took https://github.com/Metaswitch/clearwater-nginx/blob/master/clearwater-nginx/etc/monit/conf.d/nginx.monit#L8 and changed the `127.0.0.1` into `"::1"` and checked that it started up and polled successfully (instead of erroring out because check_host returned false).