Explore possibilities to integrate Kubernetes upstream e2e tests

We currently maintain our own dedicated set of end-to-end tests. During KubeCon EU 2019, I [learned](https://www.youtube.com/watch?v=kO7qJKPgxS0) that other cloud providers (like OpenStack) leverage the Kubernetes upstream tests. We should explore if we can do the same in order to reduce test duplication.

A short discovery on how to do this points at [`hack/e2e.go`](https://github.com/kubernetes/cloud-provider-openstack/blob/3c4cb3e953fc4f1ae42ae92907fc0c6b4faa60cf/contrib/post_test_hook.sh#L174-L175) which is a wrapper around [test-infra's kubetest](https://github.com/kubernetes/test-infra/tree/4289572244b4538f48072caa6664f5160f3a0580/kubetest) these days that can be installed stand-alone.