Merge pull request #35144 from pipejakob/generate-token

Automatic merge from submit-queue

New command: "kubeadm token generate"

As part of #33930, this PR adds a new top-level command to kubeadm to just generate a token for use with the init/join commands. Otherwise, users are left to either figure out how to generate a token on their own, or let `kubeadm init` generate a token, capture and parse the output, and then use that token for `kubeadm join`.

At this point, I was hoping for feedback on the CLI experience, and then I can add tests. I spoke with @mikedanese and he didn't like the original propose of `kubeadm util generate-token`, so here are the runners up:

```
$ kubeadm generate-token          # <--- current implementation
$ kubeadm generate token          # in case kubeadm might generate other things in the future?
$ kubeadm init --generate-token   # possibly as a subcommand of an existing one
```

Currently, the output is simply the token on one line without any padding/formatting:

```
$ kubeadm generate-token
1087fd.722b60cdd39b1a5f
```

CC: @kubernetes/sig-cluster-lifecycle 

**Release note**:

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``` release-note
New kubeadm command: generate-token
```

hack/verify-flags/known-flags.txt

@@ -302,6 +302,7 @@ kube-master
 kube-master
 kube-master-url
 kube-reserved
+kubeadm-path
 kubecfg-file
 kubectl-path
 kubelet-address