This makes the Kubelet config key in the ConfigMap an explicit part of
the API, so we can stop using magic key names.
As part of this change, we are retiring ConfigMapRef for ConfigMap.
Automatic merge from submit-queue. 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>.
Re-engineer the kubeadm join logic.
**What this PR does / why we need it**:
- wait for the kubelet to create `/etc/kubernetes/kubelet.conf`
- use those credentials to PATCH the node
**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: https://github.com/kubernetes/kubeadm/issues/28#issuecomment-345502933
**Special notes for your reviewer**:
/cc @kubernetes/sig-cluster-lifecycle-pr-reviews
**Release note**:
```release-note
NONE
```
