Use k3s in pipeline

Use k3s in pipeline

- replace kubespray with k3s which is
- much simpler to set up + does not need
  to be 'production-ready' to test
  deployment in pipeline
- disable addon installation
- enable k8s-plugin test run by verify-shell
- do not enable the other plugin_ tests since that
  would be too much for this CR
- pull docker images through nexus proxy to
  avoid rate limiting from dockerhub
- bump snapshot version to 0.10.2-SNAPSHOT

Issue-ID: MULTICLOUD-1513
Change-Id: I8ca40dff67ddf41153bdd2f7cfa07a22fb2b03f5
Signed-off-by: Fiete Ostkamp <fiete.ostkamp@telekom.de>