mediawiki: basic HHVM monitoring

With this change, we will add to trusty appservers:
- monitoring of the hhvm process on all HHVM appservers
- access to the admin console for hhvm locally

Change-Id: Ibbb11d37b7963a9aa0b6cf2bd379123bbc6e9a45
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>