cinder.quota.QuotaEngine.get_default function using unexpected argument

The get_default function in cinder.quota.QuotaEngine calls the _driver's
get_default function, but neither DbQuotaDriver nor NestedDbQuotaDriver
actually takes parent_project_id as an argument; see the code below.

    def get_default(self, context, resource, parent_project_id=None):
        """Get a specific default quota for a resource.

        :param parent_project_id: The id of the current project's parent,
                                  if any.
        """

        return self._driver.get_default(context, resource,
                                        parent_project_id=parent_project_id)

The NestedDbQuotaDriver get_default function is like this:

    def get_default(self, context, resource, project_id):
        """Get a specific default quota for a resource."""
        resource = super(NestedDbQuotaDriver, self).get_default(
            context, resource, project_id)

        return 0 if quota_utils.get_parent_project_id(
            context, project_id) else resource.default

The DbQuotaDriver get_default function is like this:

    def get_default(self, context, resource, project_id):
        """Get a specific default quota for a resource."""
        default_quotas = db.quota_class_get_defaults(context)
        return default_quotas.get(resource.name, resource.default)

I used the get_default function while adding a new quota item,
and the unit tests failed a lot. So I read these codes and find this part
being weired. I think this can be easily fixed, since no one seems using
this function and no unit test runs this part too.