Spawn slave environments.

        :param slave_addrs:
            List of (HOST, PORT) addresses for the slave-environments.

        :param slave_env_cls: Class for the slave environments.

        :param slave_kwargs:
            If not None, must be a list of the same size as *addrs*. Each item
            in the list containing parameter values for one slave environment.

        :param slave_mgr_cls:
            Class of the slave environment managers.

async def spawn_slaves(self, slave_addrs, slave_env_cls, slave_mgr_cls,
                           slave_kwargs=None):
        """Spawn slave environments.

        :param slave_addrs:
            List of (HOST, PORT) addresses for the slave-environments.

        :param slave_env_cls: Class for the slave environments.

        :param slave_kwargs:
            If not None, must be a list of the same size as *addrs*. Each item
            in the list containing parameter values for one slave environment.

        :param slave_mgr_cls:
            Class of the slave environment managers.
        """
        pool, r = spawn_containers(slave_addrs, env_cls=slave_env_cls,
                                   env_params=slave_kwargs,
                                   mgr_cls=slave_mgr_cls)
        self._pool = pool
        self._r = r
        self._manager_addrs = ["{}{}".format(_get_base_url(a), 0) for
                               a in slave_addrs]