Implements KEP 2000, Graceful Node Shutdown:
https://github.com/kubernetes/enhancements/tree/master/keps/sig-node/2000-graceful-node-shutdown
* Add new FeatureGate `GracefulNodeShutdown` to control
enabling/disabling the feature
* Add two new KubeletConfiguration options
* `ShutdownGracePeriod` and `ShutdownGracePeriodCriticalPods`
* Add new package, `nodeshutdown` that implements the Node shutdown
manager
* The node shutdown manager uses the systemd inhibit package, to
create an system inhibitor, monitor for node shutdown events, and
gracefully terminate pods upon a node shutdown.
Nit: remove capitalization of preferred
Remove line from kubelet and add to separate PR for easier merge
nit: dependency added to separate PR
Add check to ensure strict policy cannot be set without feature gate enabled
Topology Manager runs "none" policy by default.
Added constants for policies and updated documentation.
When node lease feature is enabled, kubelet reports node status to api server
only if there is some change or it didn't report over last report interval.
