Serve gdash.wikimedia.org on misc varnish

Add tungsten.eqiad.wmnet as a misc varnish backend and route
gdash.wikimedia.org requests to it.

This isn't blocked on completion of the Graphite migration from professor,
since Gdash doesn't actually interact with Graphite, but only constructs graph
URLs.

Needs DNS update.

Change-Id: I088a655a1eba586a482cb5aa1baee87bbb46713d
Merge "Serve gdash.wikimedia.org on misc varnish" into production