private String listeners(int node) {
            if (nodes.isCombined(node)) {
                return "EXTERNAL://localhost:0,CONTROLLER://localhost:0";
            }
            if (nodes.controllerNodes().containsKey(node)) {
                return "CONTROLLER://localhost:0";
            }
            return "EXTERNAL://localhost:0";
        }
This class manages a future which is completed with the proper value for
controller.quorum.voters once the randomly assigned ports for all the controllers are
known.
core/src/test/java/kafka/testkit/KafkaClusterTestKit.java