Enable Kubelet TLS bootstrap and NodeRestriction

* Enable bootstrap token authentication on kube-apiserver
* Generate the bootstrap.kubernetes.io/token Secret that
may be used as a bootstrap token
* Generate a bootstrap kubeconfig (with a bootstrap token)
to be securely distributed to nodes. Each Kubelet will use
the bootstrap kubeconfig to authenticate to kube-apiserver
as `system:bootstrappers` and send a node-unique CSR for
kube-controller-manager to automatically approve to issue
a Kubelet certificate and kubeconfig (expires in 72 hours)
* Add ClusterRoleBinding for bootstrap token subjects
(`system:bootstrappers`) to have the `system:node-bootstrapper`
ClusterRole
* Add ClusterRoleBinding for bootstrap token subjects
(`system:bootstrappers`) to have the csr nodeclient ClusterRole
* Add ClusterRoleBinding for bootstrap token subjects
(`system:bootstrappers`) to have the csr selfnodeclient ClusterRole
* Enable NodeRestriction admission controller to limit the
scope of Node or Pod objects a Kubelet can modify to those of
the node itself
* Ability for a Kubelet to delete its Node object is retained
as preemptible nodes or those in auto-scaling instance groups
need to be able to remove themselves on shutdown. This need
continues to have precedence over any risk of a node deleting
itself maliciously

Security notes:

1. Issued Kubelet certificates authenticate as user `system:node:NAME`
and group `system:nodes` and are limited in their authorization
to perform API operations by Node authorization and NodeRestriction
admission. Previously, a Kubelet's authorization was broader. This
is the primary security motivation.

2. The bootstrap kubeconfig credential has the same sensitivity
as the previous generated TLS client-certificate kubeconfig.
It must be distributed securely to nodes. Its compromise still
allows an attacker to obtain a Kubelet kubeconfig

3. Bootstrapping Kubelet kubeconfig's with a limited lifetime offers
a slight security improvement.
  * An attacker who obtains the kubeconfig can likely obtain the
  bootstrap kubeconfig as well, to obtain the ability to renew
  their access
  * A compromised bootstrap kubeconfig could plausibly be handled
  by replacing the bootstrap token Secret, distributing the token
  to new nodes, and expiration. Whereas a compromised TLS-client
  certificate kubeconfig can't be revoked (no CRL). However,
  replacing a bootstrap token can be impractical in real cluster
  environments, so the limited lifetime is mostly a theoretical
  benefit.
  * Cluster CSR objects are visible via kubectl which is nice

4. Bootstrapping node-unique Kubelet kubeconfigs means Kubelet
clients have more identity information, which can improve the
utility of audits and future features

Rel: https://kubernetes.io/docs/reference/command-line-tools-reference/kubelet-tls-bootstrapping/
Rel: https://github.com/poseidon/terraform-render-bootstrap/pull/185

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

@@ -22,7 +22,7 @@ systemd:
       enable: true
       contents: |
         [Unit]
-        Description=Kubelet via Hyperkube
+        Description=Kubelet
         Wants=rpc-statd.service
         [Service]
         ExecStartPre=/bin/mkdir -p /etc/kubernetes/cni/net.d
@@ -67,13 +67,14 @@ systemd:
           --anonymous-auth=false \
           --authentication-token-webhook \
           --authorization-mode=Webhook \
+          --bootstrap-kubeconfig=/etc/kubernetes/kubeconfig \
           --client-ca-file=/etc/kubernetes/ca.crt \
           --cluster_dns=${cluster_dns_service_ip} \
           --cluster_domain=${cluster_domain_suffix} \
           --cni-conf-dir=/etc/kubernetes/cni/net.d \
           --exit-on-lock-contention \
           --healthz-port=0 \
-          --kubeconfig=/etc/kubernetes/kubeconfig \
+          --kubeconfig=/var/lib/kubelet/kubeconfig \
           --lock-file=/var/run/lock/kubelet.lock \
           --network-plugin=cni \
           --node-labels=node.kubernetes.io/node \
@@ -82,6 +83,7 @@ systemd:
           %{~ endfor ~}
           --pod-manifest-path=/etc/kubernetes/manifests \
           --read-only-port=0 \
+          --rotate-certificates \
           --volume-plugin-dir=/var/lib/kubelet/volumeplugins
         ExecStop=-/usr/bin/rkt stop --uuid-file=/var/cache/kubelet-pod.uuid
         Restart=always