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
is it due to this? we should be checking for block_path right?
vault_path will always consist of atleast an empty block dir.