Set Kubelet image via kubelet.service KUBELET_IMAGE

* Write the systemd kubelet.service to use `KUBELET_IMAGE`
as the Kubelet. This provides a nice way to use systemd
dropins to temporarily override the image (e.g. during a
registry outage)

Note: Only Typhoon Kubelet images and registries are supported.

google-cloud/container-linux/kubernetes/cl/controller.yaml

@@ -52,6 +52,7 @@ systemd:
         Description=Kubelet
         Wants=rpc-statd.service
         [Service]
+        Environment=KUBELET_IMAGE=docker://quay.io/poseidon/kubelet:v1.18.2
         ExecStartPre=/bin/mkdir -p /etc/kubernetes/cni/net.d
         ExecStartPre=/bin/mkdir -p /etc/kubernetes/manifests
         ExecStartPre=/bin/mkdir -p /opt/cni/bin
@@ -90,7 +91,7 @@ systemd:
           --mount volume=var-log,target=/var/log \
           --volume opt-cni-bin,kind=host,source=/opt/cni/bin \
           --mount volume=opt-cni-bin,target=/opt/cni/bin \
-          docker://quay.io/poseidon/kubelet:v1.18.2 -- \
+          $${KUBELET_IMAGE} -- \
           --anonymous-auth=false \
           --authentication-token-webhook \
           --authorization-mode=Webhook \

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

@@ -25,6 +25,7 @@ systemd:
         Description=Kubelet
         Wants=rpc-statd.service
         [Service]
+        Environment=KUBELET_IMAGE=docker://quay.io/poseidon/kubelet:v1.18.2
         ExecStartPre=/bin/mkdir -p /etc/kubernetes/cni/net.d
         ExecStartPre=/bin/mkdir -p /etc/kubernetes/manifests
         ExecStartPre=/bin/mkdir -p /opt/cni/bin
@@ -63,7 +64,7 @@ systemd:
           --mount volume=var-log,target=/var/log \
           --volume opt-cni-bin,kind=host,source=/opt/cni/bin \
           --mount volume=opt-cni-bin,target=/opt/cni/bin \
-          docker://quay.io/poseidon/kubelet:v1.18.2 -- \
+          $${KUBELET_IMAGE} -- \
           --anonymous-auth=false \
           --authentication-token-webhook \
           --authorization-mode=Webhook \