Configure gdash to be served by nginx::site

* Add parameter docs to Gdash module.
* Use the Nginx module that was introduced in I957465edd.
* Move giant & ugly deploy_addon string literal from manifests/role/gdash.pp to
  a template.
* Get rid of Ubuntu version check. It was there to prevent professor (which is
  running Lucid) from emitting alerts, but I decided to just disable the
  alert instead of cluttering the manifests.

Change-Id: Ib5a139d909cd0b665a03ef04aa8e2a9d97f1d44f