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.
Discussion is ongoing about how to best handle dual-stack with clouds
and autodetected IPs, but there is at least agreement that people on
bare metal ought to be able to specify two explicit IPs on dual-stack
hosts, so allow that.
Several of the tests in TestNodeAddress() were no-ops because the test
code was only testing that NodeAddresses() returned all of the
expected addresses, but not testing that it was returning them in the
correct order.
The order that NodeAddresses() returns addresses in is very important,
so fix the tests to actually test it.
One existing test ("NodeIP is external") had its expectedAddresses in
the wrong order, but it seems clear from the name of the test that
this isn't actually what it expected.
Also, previously testKubeletHostname was "127.0.0.1" which ended up
interacting weirdly with the IPv4-vs-IPv6 sorting code in a way that
made some of the test results confusing if you didn't realize that
testKubeletHostname was an IPv4 address. Fix that by making it an
actual hostname instead, which then preserves the expected sorting.
