Kafka REST crashes due to KAFKA_REST_PORT defined

[jeffreymlutz]

When the KAFKA_REST_PORT variable is defined AND KAFKA_REST_LISTENERS variable is defined, then configure script exit 1/crashes. The logic should be that if KAFKA_REST_LISTENERS variable is defined and KAFKA_REST_PORT is defined, then unset KAFKA_REST_PORT variable and attempt to use the KAFKA_REST_LISTENERS variable.