Improve Kubernetes support

This is made on top of https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/merge_requests/30.

It improves support for:

• reading credentials from .kube/config or KUBECONFIG env variable,
• supporting missing docker image,
• removing helper containers to run clone/artifacts/caching support,
• adding integration tests for most common scenarios: abort, cancel, missing image, build failure,
• adds a notice to license executors/kubernetes/exec.go that this file was modified