varnish: add a php engine token

As we're going to roll HHVM into production, it will be crucial to make
the two pool easily distinguishable in the data stream analytics
processes, so that useful metrics can be extracted and performance can
be properly measured.

Change-Id: I46ff99c198892ade54eef6ffae7ebd264304a681
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>