Test non-default controller configuration

The controller can be configured to watch for resources in it's own namespace only (--all-namespaces=false) or to limit its search to resources matching a given label with `--label-selector` (#521).

Currently we cannot easily test those features because our e2e test harness first spins up the cluster with the controller binary under test and then just runs integration tests against it, while here we'd need to re-run the controller with different flags.

Let's figure out what's the simplest way to get at least some level of testing of this feature.

CC: @pfrybar

Start by reading the existing e2e test harness, how it starts the controller binary, and how integration tests connect to it. Focus on exercising --all-namespaces=false and --label-selector without assuming the default controller process can be reused. Done means the test suite covers at least some non-default controller configuration and verifies the expected resource scope.

Written by the indexing model from the issue text.