Fix how-to-retrieve-kubeconfig-from-custom-cluster

[flrichar]

Ian worked out a way to just run the entire thing inside the kubelet container for RKE1, if the customer doesn't have kubelet or jq installed and cannot for some reason, install it.

Also updates it as a secret instead of configmap, adding proper base64 -d as well.

docker exec kubelet bash -c 'kubectl --kubeconfig /etc/kubernetes/ssl/kubecfg-kube-node.yaml get secret -n kube-system full-cluster-state -ojson  | jq -r .data.\"full-cluster-state\" | base64 -d | jq -r .currentState.certificatesBundle.\"kube-admin\".config | sed -e "/^[[:space:]]*server:/ s_:.*_: \"https://127.0.0.1:6443\"_" > kubeconfig.yaml'