varnish: make varnish::instance not depend on ganglia

We may have situations where we don't have either ganglia or gmond, so
we don't want to have varnish::instance depend on it; also, the new
syntax using the spaceship operator and tags should be easier to read
and maintain. This is a prerequisite to fix bug #73263

Change-Id: I6ebf100e64cb432bb073831131c6be62dfac5695
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>