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
@TheSriram  There is no block_path in the disk driver. Blocks are stored directly under the vault path.

We're having trouble figuring out what is not working for @jc7998 as there is a unit test here that passes just fine, but his API test does not. We're not sure where the disconnect is yet.