Update Kubernetes from v1.11.3 to v1.12.1

* Mount an empty dir for the controller-manager to work around
https://github.com/kubernetes/kubernetes/issues/68973
* Update coreos/pod-checkpointer to strip affinity from
checkpointed pod manifests. Kubernetes v1.12.0-rc.1 introduced
a default affinity that appears on checkpointed manifests; but
it prevented scheduling and checkpointed pods should not have an
affinity, they're run directly by the Kubelet on the local node
* https://github.com/kubernetes-incubator/bootkube/issues/1001
* https://github.com/kubernetes/kubernetes/pull/68173

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

@@ -93,7 +93,7 @@ storage:
       contents:
         inline: |
           KUBELET_IMAGE_URL=docker://k8s.gcr.io/hyperkube
-          KUBELET_IMAGE_TAG=v1.11.3
+          KUBELET_IMAGE_TAG=v1.12.1
     - path: /etc/sysctl.d/max-user-watches.conf
       filesystem: root
       contents:
@@ -111,7 +111,7 @@ storage:
             --volume config,kind=host,source=/etc/kubernetes \
             --mount volume=config,target=/etc/kubernetes \
             --insecure-options=image \
-            docker://k8s.gcr.io/hyperkube:v1.11.3 \
+            docker://k8s.gcr.io/hyperkube:v1.12.1 \
             --net=host \
             --dns=host \
             --exec=/kubectl -- --kubeconfig=/etc/kubernetes/kubeconfig delete node $(hostname)