Add older k3s versions for 1.14 and 1.15

We still support Kubernetes 1.14 (until December) and 1.15 (until ??) but it would be good to have a unified test interface to run in a build matrix. The idea is that as long as service:command is not overridden, the user should not see a difference.

Also adds a retry mechanism when loading the kubeconfig because it's possible the web server starts before k3s.