Run k3s manually instead

Workaround issue with k3s and kubectl 1.11.10. I guess with kubectl <
1.1.10 the --username and --password parameters weren't being recognised
or something.

We could have extracted /etc/rancher/k3s/k3s.yaml from the linked k3s
docker service with:

`docker cp k3s:/etc/rancher/k3s/k3s.yaml kubeconfig.yaml`

but the running container does not have access to the linked services,
only the job container
(https://gitlab.com/gitlab-org/gitlab-runner/merge_requests/1041/diffs)