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

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

@@ -128,8 +128,7 @@ storage:
             docker://quay.io/poseidon/kubelet:v1.18.0 \
             --net=host \
             --dns=host \
-            -- \
-            /usr/local/bin/kubectl -- --kubeconfig=/etc/kubernetes/kubeconfig delete node $(hostname | tr '[:upper:]' '[:lower:]')
+            --exec=/usr/local/bin/kubectl -- --kubeconfig=/etc/kubernetes/kubeconfig delete node $(hostname | tr '[:upper:]' '[:lower:]')
 passwd:
   users:
     - name: core

digital-ocean/container-linux/kubernetes/cl/worker.yaml

@@ -134,5 +134,4 @@ 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)

google-cloud/container-linux/kubernetes/workers/cl/worker.yaml

@@ -128,8 +128,7 @@ 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

google-cloud/fedora-coreos/kubernetes/workers/fcc/worker.yaml

@@ -87,7 +87,7 @@ systemd:
         Type=oneshot
         RemainAfterExit=true
         ExecStart=/bin/true
-        ExecStop=/bin/bash -c '/usr/bin/podman run --volume /etc/kubernetes:/etc/kubernetes:ro,z --entrypoint /usr/local/bin/kubectl quay.io/poseidon/kubelet:v1.18.0 kubectl --kubeconfig=/etc/kubernetes/kubeconfig delete node $HOSTNAME'
+        ExecStop=/bin/bash -c '/usr/bin/podman run --volume /etc/kubernetes:/etc/kubernetes:ro,z --entrypoint /usr/local/bin/kubectl quay.io/poseidon/kubelet:v1.18.0 --kubeconfig=/etc/kubernetes/kubeconfig delete node $HOSTNAME'
         [Install]
         WantedBy=multi-user.target
 storage: