Make Parsoid Varnish backend IP site dependent

It was hardcoded to 10.2.1.28, the VIP in Tampa. So even now that we've
switched to eqiad, Parsoid is still being served out of Tampa. This
change makes the Varnish backend IP depend on the datacenter.

Change-Id: I27668a2b651a9786d394be77ac5f7ce650171cbc
Merge "Make Parsoid Varnish backend IP site dependent" into production