feat: delete block endpoint

Blocks can now be deleted, if they have no references. A status
code of 204 is then returned.
A Constraint Error is raised by the metadata driver if there
are references, and 412 is returned as status code.

Endpoint:
    DELETE /v1.0/vaults/{vault}/blocks/{blockid}
This logic is currently in place in the controller: https://github.com/rackerlabs/deuce/pull/125/files#diff-116594504b83db4b6d1aecb67c11b5b4R153