Define DeleteVolume behavior with snapshots

Spec is not clear on what should happen when on a DeleteVolume call when
the volume has snapshots.

This patch clarifies the situation by explicitly mentioning that the
operation should complete and the snapshots should still be operational.

Closes: container-storage-interface#346

Author: Akrog

spec.md

@@ -1131,6 +1131,8 @@ This RPC will be called by the CO to deprovision a volume.
 This operation MUST be idempotent.
 If a volume corresponding to the specified `volume_id` does not exist or the artifacts associated with the volume do not exist anymore, the Plugin MUST reply `0 OK`.
 
+CSI plugins MUST treat volumes independent from their snapshots.  Controller Plugin MUST support deleting a volume without affecting its existing snapshots, which MUST still be fully operational and acceptable as sources for new volumes as well as appear on `ListSnapshot` calls.
+
 ```protobuf
 message DeleteVolumeRequest {
   // The ID of the volume to be deprovisioned.