hostpath: Implement ValidateVolumeCapabilities

[darkowlzz]

This implements a basic version of ValidateVolumeCapabilities for
hostpath driver and removes this from the skip tests list in the e2e
tests.

Running the e2e tests locally always failed because it was skipped for travisCI environment only.

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[pohly]

This looks like a useful enhancement. However, the hostpath driver is about to be moved to a separate repo (see kubernetes-csi/csi-driver-host-path#1), so the PR will have to be moved there once that PR is merged.

[pohly]

/hold

I'm holding this because we don't want to add any further changes here. It's still ready for review.

[pohly]

@darkowlzz Please move this PR over to the csi-driver-host-path repo. The code was merged into master of that repo.

[darkowlzz]

Moved to kubernetes-csi/csi-driver-host-path#5.