HHVM enabler for application servers

Adds a 'hhvm' boolean parameter to selectively install hhvm on
application server classes.  Default to 'false' since it is not ready
yet for production usage (also added a fail() to be safe).

While at it, disable installing hhvm on labs since it is libmemcached
dependency conflict with the one from wikimedia-task-appserver.

Change-Id: I34ccd7d9bbecbdbb2f7ac792d502b694bbeeb202
Merge "HHVM enabler for application servers" into production