code: def set_credentials(self, username, password=None, region=None,
              tenant_id=None, authenticate=False):
          
          self.api_key = None
          super(RaxIdentity, self).set_credentials(username, password=password,
                  region=region, tenant_id=tenant_id, authenticate=authenticate)
docs: Sets the username and password directly. Because Rackspace auth uses
          the api_key, make sure that any old values are cleared.