add varnish::kafka::statsv

Add a varnishkafka instance on the bits varnishes that logs requests to
bits.wikimedia.org/statsv/*. This will be used for reporting performance metrics from client
side code.

This also includes a slight refactore of cache.pp varnish::kafka classes
so to abstract out default stuff we want happening for every varnishkafka::instance.

Change-Id: I3d74c9da11c979a9f3cdcb4ebf1b34d225d56061