varnish: sort bits esams backends

It currently oscillates between pmtpa/eqiad & eqiad/pmtpa, forcing a VCL
reload on half of the puppet runs.

Fix this by sorting the list on cache.pp, rather than sorting the
backends list in the erb template; it would be equivalent right now, but
we might use the "fallback" director at some point and backend order
matters with that director, so let's have our erbs be future-proof.

Change-Id: I1b542fa77b323a11e4ba1a34a4753d270361bef9