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Secure Kubelet's componentconfig defaults while maintaining CLI compatibility
This updates the Kubelet's componentconfig defaults, while applying the legacy defaults to values from options.NewKubeletConfiguration(). This keeps defaults the same for the command line and improves the security of defaults when you load config from a file.
See: https://github.com/kubernetes/kubernetes/issues/53618
See: https://github.com/kubernetes/kubernetes/pull/53833#discussion_r166669931
Also moves EnableServer to KubeletFlags, per @tallclair's comments on #53833.
We should find way of generating documentation for config file defaults, so that people can easily look up what's different from flags.
```release-note
Action required: Default values differ between the Kubelet's componentconfig (config file) API and the Kubelet's command line. Be sure to review the default values when migrating to using a config file.
```
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