Explore possibilities to integrate Kubernetes upstream e2e tests

[timoreimann]

We currently maintain our own dedicated set of end-to-end tests. During KubeCon EU 2019, I learned 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 which is a wrapper around test-infra's kubetest these days that can be installed stand-alone.

[timoreimann]

One approach worth considering is to leverage Cluster API (and the DO provider) to bring up the infrastructure needed to test CCM on DO resources.