Merge pull request #46254 from mtaufen/dkcfg

Automatic merge from submit-queue (batch tested with PRs 50016, 49583, 49930, 46254, 50337)

Alpha Dynamic Kubelet Configuration

Feature: https://github.com/kubernetes/features/issues/281

This proposal contains the alpha implementation of the Dynamic Kubelet Configuration feature proposed in ~#29459~ [community/contributors/design-proposals/dynamic-kubelet-configuration.md](https://github.com/kubernetes/community/blob/master/contributors/design-proposals/dynamic-kubelet-configuration.md). 

Please note:
- ~The proposal doc is not yet up to date with this implementation, there are some subtle differences and some more significant ones. I will update the proposal doc to match by tomorrow afternoon.~
- ~This obviously needs more tests. I plan to write several O(soon). Since it's alpha and feature-gated, I'm decoupling this review from the review of the tests.~ I've beefed up the unit tests, though there is still plenty of testing to be done.
- ~I'm temporarily holding off on updating the generated docs, api specs, etc, for the sake of my reviewers 😄~ these files now live in a separate commit; the first commit is the one to review.

/cc @dchen1107 @vishh @bgrant0607 @thockin @derekwaynecarr 

```release-note
Adds (alpha feature) the ability to dynamically configure Kubelets by enabling the DynamicKubeletConfig feature gate, posting a ConfigMap to the API server, and setting the spec.configSource field on Node objects. See the proposal at https://github.com/kubernetes/community/blob/master/contributors/design-proposals/dynamic-kubelet-configuration.md for details.
```

test/integration/etcd/etcd_storage_path_test.go

@@ -343,6 +343,7 @@ var ephemeralWhiteList = createEphemeralWhiteList(
 	gvr("", "v1", "rangeallocations"),     // stored in various places in etcd but cannot be directly created
 	gvr("", "v1", "componentstatuses"),    // status info not stored in etcd
 	gvr("", "v1", "serializedreferences"), // used for serilization, not stored in etcd
+	gvr("", "v1", "nodeconfigsources"),    // subfield of node.spec, but shouldn't be directly created
 	gvr("", "v1", "podstatusresults"),     // wrapper object not stored in etcd
 	// --