This extends the Kubelet to create and periodically update leases in a
new kube-node-lease namespace. Based on [KEP-0009](https://github.com/kubernetes/community/blob/master/keps/sig-node/0009-node-heartbeat.md),
these leases can be used as a node health signal, and will allow us to
reduce the load caused by over-frequent node status reporting.
- add NodeLease feature gate
- add kube-node-lease system namespace for node leases
- add Kubelet option for lease duration
- add Kubelet-internal lease controller to create and update lease
- add e2e test for NodeLease feature
- modify node authorizer and node restriction admission controller
to allow Kubelets access to corresponding leases
This makes the Kubelet config key in the ConfigMap an explicit part of
the API, so we can stop using magic key names.
As part of this change, we are retiring ConfigMapRef for ConfigMap.
nodes should only be able to create TokenRequests if:
* token is bound to a pod
* binding has uid and name
* the pod exists
* the pod is running on that node
