mediawiki: collect apc variables via diamond

We created a class that is a basic framework for setting up monitoring
endpoints in mediawiki, and that will allow to add other endpoints
easily in the future. This first patch will only enable the monitoring
virtual host on one server; after testing, the safeguards will be
removed and it will be deployed to all mediawikis using the
mediawiki::web class.

Change-Id: I4f8629fbc9462d84a86db4c0da57b366e7a4cbc4
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>