'Deletes a credential.
For a full list of available parameters, please refer to the official
API reference:
http://developer.openstack.org/api-ref/identity/v3/#delete-credential'

def delete_credential(self, credential_id):

    (resp, body) = self.delete(('credentials/%s' % credential_id))
    self.expected_success(204, resp.status)
    return rest_client.ResponseBody(resp, body)
