Separate HHVM app servers backend.

This creates a separate backend for regular app servers with HHVM.
If the request has a certain cookie ('hhvm' for now), the request
is sent to the HHVM app servers, provided it's not already assigned
a different backend like the API or testwiki.

The cache for responses coming from HHVM is separated from the one of
standard Zend appservers.

If a request gets restarted e.g. in vcl_error, the normal Zend
app servers are used instead, and the response in this case
is not cached.

The LVS service has been assigned an IP, but needs to be setup
seperately.

Tested in beta.

Change-Id: Ic9531496b8259bb32b80e3e613fb1df4687a036c