Automatic merge from submit-queue

kubeadm: Implementing the kubeconfig phase fully

**What this PR does / why we need it:**
This contains implementation of kubeconfig phases in kubeadm, which is part of the wider effort of implementing phases in kubeadm, previously in alpha stage.

The original proposal for this activity can be found [here](https://github.com/kubernetes/kubeadm/pull/156/files) and related comments.

Kubeadm phase implementation checklist is defined [here](https://github.com/kubernetes/kubeadm/issues/267)

Common implementation guidelines and principles for all phases are defined [here](https://docs.google.com/document/d/1VQMyFIVMfRGQPP3oCUpfjiWtOr3pLxp4g7cP-hXQFXc/edit?usp=sharing)

This PR implements:

- [x] kubeadm phase kubeconfig
  - [x] kubeadm phase kubeconfig all
  - [x] kubeadm phase kubeconfig admin
  - [x] kubeadm phase kubeconfig kubelet
  - [x] kubeadm phase kubeconfig scheduler
  - [x] kubeadm phase kubeconfig controller-manager
  - [x] kubeadm phase kubeconfig user

**Which issue this PR fixes:**
https://github.com/kubernetes/kubeadm/issues/350

**Special notes for your reviewer:**

This PR implements the second phases of kubeadm init; implementation of this PR follow the same approach already used for #48196 (cert phases).

Please note that:
- the API - phase\kubeconfig.go - is now totally free by any UX concerns, and implements only the core logic for kubeconfig generation.
- the UX - cmd\phase\kubeconfig.go - now takes charge of UX commands and kubeadm own's rules for kubeconfig files in /etc/kubernetes folder (e.g. create only if not already exists)
- The PR includes also a fix for a regression on a unit test for phase certs introduced by #48594 and few minor code changes in phase certs introduced to avoid code duplication between the two phases.