Automatic merge from submit-queue

kubeadm: Default to v1.6.0 stable in offline scenarios in beforehand

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

In offline scenarios, kubeadm will fallback to the latest well-known version.
This PR bumps that to v1.6. We can merge now, and in the small gap between the merge of this PR and the actual v1.6 release, kubeadm devs will have to explicitely set k8s version.

**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**:

**Release note**:

```release-note
NONE
```
@jbeda