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Add mountpoint as CRI image filesystem storage identifier.
Fixes https://github.com/kubernetes/kubernetes/issues/57356.
This PR changes CRI to use mountpoint as storage identifier. See https://github.com/kubernetes/kubernetes/issues/57356#issuecomment-363608733.
Note that:
1) This doesn't work with devicemapper for now. Please feel free to propose change for device mapper, we can discuss more about this after this first version is merged. @mrunalp @runcom
2) `mountpoint` is added as new field in `StorageIdentifier` now. After https://github.com/kubernetes/kubernetes/pull/58973 is merged, we can remove the UUID field in `v1alpha2`.
/cc @yujuhong @feiskyer @yguo0905 @dashpole @mikebrow @abhi @kubernetes/sig-node-api-reviews
**Release note**:
```release-note
CRI starts using moutpoint as image filesystem identifier instead of UUID.
```
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