Don't default ADVERTISED_PORT if listeners are configured

If configuring listeners (and/or advertised.listeners) to support a
configuration with both PLAINTEXT and SSL endpoints[1], you must not set either
of advertised.{host,port}, otherwise the broker will only create the PLAINTEXT
endpoint.

advertised.host won't be defaulted unless another env var is set, but without
this patch there was no way to prevent advertised.port from being defaulted when
not present.

[1]: http://docs.confluent.io/2.0.0/kafka/incremental-security-upgrade.html

start-kafka.sh

@@ -3,7 +3,9 @@
 if [[ -z "$KAFKA_PORT" ]]; then
     export KAFKA_PORT=9092
 fi
-if [[ -z "$KAFKA_ADVERTISED_PORT" ]]; then
+if [[ -z "$KAFKA_ADVERTISED_PORT" && \
+  -z "$KAFKA_LISTENERS" && \
+  -z "$KAFKA_ADVERTISED_LISTENERS" ]]; then
     export KAFKA_ADVERTISED_PORT=$(docker port `hostname` $KAFKA_PORT | sed -r "s/.*:(.*)/\1/g")
 fi
 if [[ -z "$KAFKA_BROKER_ID" ]]; then