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kubeadm: fix runtime.ImageExists API
**What this PR does / why we need it**:
runtime.ImageExists returns error when underlying 'inspect' command
fails. This makes ImagePullCheck to fail as it doesn't expect
runtime.ImageExists to return an error even if image doesn't exist.
**Which issue(s) this PR fixes**:
Fixes [kubeadm issue 1024](https://github.com/kubernetes/kubeadm/issues/1024)
**Release note**:
```release-note
NONE
```
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