Fix delete-node.service kubectl service exec's

* Fix delete-node service that runs on worker (cloud-only)
shutdown to delete a Kubernetes node. Regressed in #669
(unreleased)
* Use rkt `--exec` to invoke kubectl binary in the kubelet
image
* Use podman `--entrypoint` to invoke the kubectl binary in
the kubelet image

aws/container-linux/kubernetes/workers/cl/worker.yaml

@@ -130,8 +130,7 @@ storage:
             docker://quay.io/poseidon/kubelet:v1.18.0 \
             --net=host \
             --dns=host \
-            -- \
-            /usr/local/bin/kubectl --kubeconfig=/etc/kubernetes/kubeconfig delete node $(hostname)
+            --exec=/usr/local/bin/kubectl -- --kubeconfig=/etc/kubernetes/kubeconfig delete node $(hostname)
 passwd:
   users:
     - name: core