hhvm: add hhvm_health Ganglia module and hhvmadm script

* Report the metrics provided by the /check-health endpoint of the HHVM admin
  server to Ganglia.
* Provision a simple shell script, 'hhvmadm', that helps with calling endpoints
  of the admin server from the shell.

Change-Id: I79d247d2702cc131224f2c3433fd784567cf66b4