Automatic merge from submit-queue (batch tested with PRs 63624, 59847). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

explicit kubelet config key in Node.Spec.ConfigSource.ConfigMap

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.


```release-note
You must now specify Node.Spec.ConfigSource.ConfigMap.KubeletConfigKey when using dynamic Kubelet config to tell the Kubelet which key of the ConfigMap identifies its config file.
```