Bugfix disk driver vault deletion

- Updated Disk Storage Driver to check whether or not the Vault was empty before deleting it
- Updated Vault Controller to return 204 on success or  412 on failure when deleting a Vault
- Added a Unit Test to specifically check the Vault Deletion functionality
There is no functionality to delete a block at this time so it's there for when we do have it so we can enable the rest of the test