Returns a (cached) set of the distinguished names in self._group_infos.

def get_group_dns(self):
        """
        Returns a (cached) set of the distinguished names in self._group_infos.
        """
        if self._group_dns is None:
            group_infos = self._get_group_infos()
            self._group_dns = set(group_info[0] for group_info in group_infos)

        return self._group_dns