Directive for registering a RDBM based repository.

def rdb_repository(_context, name=None, make_default=False,
                   aggregate_class=None, repository_class=None,
                   db_string=None, metadata_factory=None):
    """
    Directive for registering a RDBM based repository.
    """
    cnf = {}
    if not db_string is None:
        cnf['db_string'] = db_string
    if not metadata_factory is None:
        cnf['metadata_factory'] = metadata_factory
    _repository(_context, name, make_default,
                aggregate_class, repository_class,
                REPOSITORY_TYPES.RDB, 'add_rdb_repository', cnf)