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