Automatic merge from submit-queue (batch tested with PRs 45309, 45376)

kubeadm: Fix the tainting of the master node

**What this PR does / why we need it**:

Currently, kubeadm unintentionally removes Taints set with the kubelet flag `--register-with-taints` when creating a new Taint slice instead of appening the existing one. This PR fixes that behavior.

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #

**Special notes for your reviewer**:

This should be a cherrypick candidate for v1.6 cc @enisoc 

**Release note**:

```release-note
kubeadm: Fix invalid assign statement so it is possible to register the master kubelet with other initial Taints
```
@jbeda @dmmcquay @mikedanese @krousey PTAL, very straightforward PR