Make "varnish" be the default instance name, instead of the hostname

Using the hostname in the metric name was a bad idea in hindsight,
as it doesn't allow for easy comparison and aggregation of metrics
between hosts in Ganglia and Torrus.

Change-Id: I4f2ecb61ca5fd9bfaf14a9c87edea8f4ec527414
Merge "Make "varnish" be the default instance name, instead of the hostname" into production