add (and use) shell_exports() and apache::vars

* Add useful shell_exports() from MediaWiki-Vagrant; renders a Puppet hash
  into shell exports
* Not confined to Apache module or a template because it will be useful
  in other places.
* Add apache::env for declaring Apache envvars.
* Use in MediaWiki module

Change-Id: I84183eb61df3921d9e2f2034648e7d61351f1a5d