Create a random director for non-tier1 backends as well

Tier 2 backend Varnish instances talk to tier 1 backends,
and therefore popular (uncacheable) URLs might still get
concentrated on a single tier 1 host.

Change-Id: I1cacb398549583900d7453977dc44faba14ffe8d
Merge "Create a random director for non-tier1 backends as well" into production