Add resource for server metrics site

* Configure an Nginx site, bound to lo, that serves server metrics on
  /nginx_status.
* Uses http://wiki.nginx.org/HttpStubStatusModule, which is included and
  enabled in the Debian package.
* This is how the Diamond Nginx collector gets Nginx metrics:
  https://github.com/BrightcoveOS/Diamond/wiki/collectors-NginxCollector
* A follow-up patch for the Diamond module will add an nginx collector
  that utilizes this resource.

Change-Id: I9450b8cf05cd61416035c4fcde56aa93285243e1