Varnish: switch upload to eqiad Swift cluster

Switch upload tier 1 Varnishes from having the pmtpa Swift cluster
(ms-fe.pmtpa.wmnet) as backend o the eqiad one. Deliberately switch to
"eqiad" instead of $mw_primary, as we don't consider Swift as
georeplicated the same way as MediaWiki, for now.

Change-Id: I83fe4c664aad01b44433cace379aea9e65e670ce