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
If there's stuff under the vault other than blocks/, we should not delete it.  

If we have to do it, Swift driver should mapping the same behavior (i.e., rmtree())