Update Kubernetes from v1.12.2 to v1.12.3

* https://github.com/kubernetes/kubernetes/blob/master/CHANGELOG-1.12.md/#v1123

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

@@ -94,7 +94,7 @@ storage:
       contents:
         inline: |
           KUBELET_IMAGE_URL=docker://k8s.gcr.io/hyperkube
-          KUBELET_IMAGE_TAG=v1.12.2
+          KUBELET_IMAGE_TAG=v1.12.3
     - path: /etc/sysctl.d/max-user-watches.conf
       filesystem: root
       contents:
@@ -112,7 +112,7 @@ storage:
             --volume config,kind=host,source=/etc/kubernetes \
             --mount volume=config,target=/etc/kubernetes \
             --insecure-options=image \
-            docker://k8s.gcr.io/hyperkube:v1.12.2 \
+            docker://k8s.gcr.io/hyperkube:v1.12.3 \
             --net=host \
             --dns=host \
             --exec=/kubectl -- --kubeconfig=/etc/kubernetes/kubeconfig delete node $(hostname)