For sharding: we get the http port number from the Endpoints of the
Varnish service. From the PodSpec and -Status we get the host
name and the PodIP.

Initially to publish workqueue metrics.

Workaround for Varnish issue #2880:
https://github.com/varnishcache/varnish-cache/issues/2880

Refactor the test sources a bit while we're here.

Also shorten some names that predictably include '.' -- Service
backends and Host strings.

'compare' can be one of equal, not-equal, match or non-match.

'value' is either a fixed string for (in)equality tests, or a
regex to test for (non-)match.

This makes it possible to set values for the invocation via env
variables in the pod template, in particular to use PROXY.

This has required the introduction of pkg/interfaces, defining an
interface for Service Event generation, so that the Varnish controller
can hold a reference to IngressController, without introducing
import cycles for pkg/varnish and pkg/controller.