'List all projects in the subtree of a given project.
:param project_id: the driver will get the subtree under
this project.
:returns: a list of project_refs or an empty list
:raises keystone.exception.ProjectNotFound: if project_id does not
exist'

@abc.abstractmethod
def list_projects_in_subtree(self, project_id):

    raise exception.NotImplemented()
