Test with two SASL_SSL listeners with different JAAS contexts

rsivaram suggested the following in https://github.com/apache/kafka/pull/2406

I think this feature allows two SASL_SSL listeners, one for external and one for internal and the two can use different mechanisms and different JAAS contexts. That makes the multi-mechanism configuration neater. I think it will be useful to have an integration test for this, perhaps change SaslMultiMechanismConsumerTest.

And my reply:

I think it's a bit tricky to support multiple listeners in KafkaServerTestHarness. Maybe it's easier to do the test you suggest in MultipleListenersWithSameSecurityProtocolTest.