code: def start_external_service(self, service_name, conf=None):
          
          if service_name in self._external_services:
              ser = self._external_services[service_name]
              service = ser(service_name, conf=conf, bench=self.bench)
              try:
                  service.start()
              except PluginException:
                  self.logger.exception("Starting service %s caused an exception!", service_name)
                  raise PluginException("Failed to start external service {}".format(service_name))
              self._started_services.append(service)
              setattr(self.bench, service_name, service)
          else:
              self.logger.warning("Service %s not found. Check your plugins.", service_name)
docs: Start external service service_name with configuration conf.
  
          :param service_name: Name of service to start
          :param conf:
          :return: nothing