Set cluster_tier for bits caches

templates/varnish/bits.inc.vcl.erb has (for a very long
time) had a vcl_miss block that's only to be included
in tier-1 caches (currently that's eqiad).  This was
limited to the tier-1 caches with:

    -<% if site == "pmtpa" or site == "eqiad" -%>

Back on 2014-01-28, commit 30eb53e9 replaced that line with:

    <% if vcl_config.fetch("cluster_tier", "1") == "1" -%>

In general most of our multi-tier cache configs have this
variable (text, mobile, upload) set in their vcl_config
in manifests/role/cache.pp, but bits did not.

The result of this commit was that the tier-2 bits caches
(ulsfo, esams) started including the tier-1-only vcl_miss block.
That block references the backend test_wikipedia, which isn't
defined for those tier-2 configs, so VCL reload failed
and the faulty configuration was never applied on those
hosts.

However, due to the infelicities of puppet state tracking
and how we do VCL reloads, this was just a single puppet
run failure in isolation.  After that we've had no notice
of this condition since then (Jan 2014), and the VCL on
these hosts has effectively been frozen, preventing any
future changes from affecting them.  The problem would
only be indicated by a single, isolated puppet run failure
when a bits-VCL-affecting commit was merged, then back
to pretending everything is fine.

I've audited the relevant bits-VCL-affecting commits since
the breaking commit as best I can, and I don't believe any
since then were of significant runtime consequence, but I
can't be 100% sure.

Change-Id: I654f58f493cba78f51e5a9570b8d0fe90a4ee9ae