Automatic merge from submit-queue
kubeadm: Implement 'kubeadm config'
**What this PR does / why we need it**:
Implements a `kubeadm config` command for viewing the current kubeadm configuration stored as a ConfigMap in the cluster and creating that configuration for v1.7- users. kubeadm v1.8+ handles the creation of this ConfigMap at init time, but v1.7 users have to create it themselves with this command in order to be able to preserve the same config after the upgrade.
**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #
fixes: https://github.com/kubernetes/kubeadm/issues/406
**Special notes for your reviewer**:
**Release note**:
```release-note
Adds a new `kubeadm config` command that lets users tell `kubeadm upgrade` what kubeadm configuration to use and lets users view the current state.
```
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