Rename all webrequest varnishkafka instances as 'webrequest'

This standardizes the varnishkafka conf and stats files to have the
same name across all webrequest (frontend + bits) instances.
Previously, frontend confs were named frontend.conf, and bits
confs were named $::hostname.conf.  This makes it easier to
know which varnish instance is logging 'webrequests' and
also which stats.json files one should look in for stats about
webrequest varnishkafkas.

Change-Id: I0337ede6cd9c9022a5f4d6ebc05f032ddb428bef