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**:

<!--  Steps to write your release note:
1. Use the release-note-* labels to set the release note state (if you have access) 
2. Enter your extended release note in the below block; leaving it blank means using the PR title as the release note. If no release note is required, just write `NONE`. 
-->

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

hack/make-rules/test-kubeadm-cmd.sh

@@ -0,0 +1,32 @@
+#!/bin/bash
+
+# Copyright 2016 The Kubernetes Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+set -o errexit
+set -o nounset
+set -o pipefail
+
+KUBE_ROOT=$(dirname "${BASH_SOURCE}")/../..
+source "${KUBE_ROOT}/hack/lib/init.sh"
+
+KUBEADM_PATH="${KUBEADM_PATH:=$(kube::realpath "${KUBE_ROOT}")/cluster/kubeadm.sh}"
+
+# If testing a different version of kubeadm than the current build, you can
+# comment this out to save yourself from needlessly building here.
+make -C "${KUBE_ROOT}" WHAT=cmd/kubeadm
+
+make -C "${KUBE_ROOT}" test \
+  WHAT=cmd/kubeadm/test \
+  KUBE_TEST_ARGS="--kubeadm-path '${KUBEADM_PATH}'"

hack/make-rules/test.sh

@@ -35,6 +35,7 @@ kube::test::find_dirs() {
           -path './_artifacts/*' \
           -o -path './_output/*' \
           -o -path './_gopath/*' \
+          -o -path './cmd/kubeadm/test/*' \
           -o -path './contrib/podex/*' \
           -o -path './output/*' \
           -o -path './release/*' \