Add a 2nd 200GB persistent storage backend per SSD

The eqiad (and ulsfo) Varnish servers were mistakenly configured
with 100GB storage per SSD, instead of 300GB. Because we can't
afford to drop the caches now during the migration, we'll add
a second one and merge them one by one later.

Change-Id: I7fa968a34bed9a29570da80f2f4425d0ab20e2e5
Merge "Add a 2nd 200GB persistent storage backend per SSD" into production