Get address of the slave environment manager with the smallest
        number of agents.

async def _get_smallest_env(self):
        """Get address of the slave environment manager with the smallest
        number of agents.
        """
        async def slave_task(mgr_addr):
            r_manager = await self.env.connect(mgr_addr, timeout=TIMEOUT)
            ret = await r_manager.get_agents(addr=True)
            return mgr_addr, len(ret)

        sizes = await create_tasks(slave_task, self.addrs, flatten=False)
        return sorted(sizes, key=lambda x: x[1])[0][0]