d4e8a6cb4c
Automatic merge from submit-queue
kubeadm: don't duplicate master taint if it already exists.
**What this PR does / why we need it**:
This helps enable a graceful upgrade/downgrade process between 1.6.x and 1.7.x kubeadm clusters (although no guarantees outside of that range) by doing:
$ kubeadm init --kubernetes-version <version> --skip-preflight-checks
Without this change, the command fails with an error that the node taint is duplicated.
This is part of https://github.com/kubernetes/kubeadm/issues/278
**Release note**:
```release-note
NONE
```
Fixes: https://github.com/kubernetes/kubeadm/issues/288
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