HAT: depool faulty servers, monitor hhvm rendering

Given we have had a few issues with hhvm crashing badly, we do some
mitigations:

* depool servers until at least 30% of the cluster is alive
* monitor an url served by the PHP engine, and not directly by apache

Change-Id: I77cd3510a5d22a96ff4602eca7b5b539a2a36ee1
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>