hhvm: abstract out backports to a hhvm module

The HHVM package we use requires liboost 1.0.49 which is not available
in Ubuntu Precise.  Ori added a third party ppa in
applicationserver::hhvm , I need the same setup for the contint slaves.

This patch creates a new hhvm puppet module which hold the backport
logic in the standalone hhvm::backports.  The hhvm class install the
package and require the backport to be installed.

In applicationserver, simply include the class, keeping the
apache_mod_fastcgi_for_hhvm requirement.

Also added hhvm to contint slaves.

Change-Id: I3bab6e0ec5f1fc769ee0e2659d73ded82c6980ed