Automatic merge from submit-queue (batch tested with PRs 39199, 37273, 29183, 39638, 40199)
Refactor/improve the kubeadm generation of certificates
**What this PR does / why we need it**:
Continues to refactor/improve kubeadm towards beta.
**Special notes for your reviewer**:
Modify the certs that are generated; generate on demand (if not exist) and only four files instead of eight previously. Basically implements what has been discussed so far in https://github.com/kubernetes/kubeadm/pull/100
**Release note**:
```release-note
NONE
```
cc @mikedanese @pires @lukemarsden @errordeveloper @dgoodwin @roberthbailey
0275ca0490