Fix response from snapshot create stub

A change from Oleg highlighted that the current snapshot create
stub is broken because it passes a full volume for the volume_id
paramter.

Fix the stub and the api_samples output, but also add unit test
checks which would have caught this.

Change-Id: Id006e3995c7696aa8f061a2b96123ea27e4b6d3f
Co-authored-by: Oleg Bondarev <obondarev@mirantis.com>
Merge "Fix response from snapshot create stub"