kubeadm: Fix the tainting of the master node

[luxas]

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:

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

[k8s-reviewable]

This change is 

[k8s-cherrypick-bot]

Removing label cherrypick-candidate because no release milestone was set. This is an invalid state and thus this PR is not being considered for cherry-pick to any release branch. Please add an appropriate release milestone and then re-add the label.

[dmmcquay]

/lgtm

[k8s-github-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dmmcquay, luxas

Needs approval from an approver in each of these OWNERS Files:

• cmd/kubeadm/OWNERS [luxas]

You can indicate your approval by writing /approve in a comment
You can cancel your approval by writing /approve cancel in a comment

[k8s-github-robot]

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

[mikedanese]

/lgtm

[k8s-cherrypick-bot]

Commit found in the "release-1.6" branch appears to be this PR. Removing the "cherrypick-candidate" label. If this is an error find help to get your PR picked.