Automatic merge from submit-queue (batch tested with PRs 62441, 66702, 67254, 67421, 65309). 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>.

[kubeadm] Apply means to search for existing KubeConfig files in standard locations across kubeadm

**What this PR does / why we need it**:
Apply means to search for existing KubeConfig files in standard locations across kubeadm.

Tasklist:

- [x] Use the Join of Dir and File name instead of DefaultKubeConfig
- [x] Refactor token.go::findExistingKubeConfig() into a utility in cmd/kubeadm/app/cmd/util
- [x] Use the utility in config, upgrade and some of the phases commands that use --kubeconfig

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
ref [kubernetes/kubeadm/#805](https://github.com/kubernetes/kubeadm/issues/805)

**Special notes for your reviewer**:
/assign @neolit123 

**Release note**:

```release-note
NONE
```