`--kafka-container` argument is ignored

[ble]

Reproduction steps:

(I'm using pipenv to run the script; your python environment management technique may vary 😀)

# Omit --kafka-container argument and write output to `without-arg.yaml`:
pipenv run python3 kafka_docker_composer.py --shared-mode -b 3 -c 1 -s 0 -C 0 --docker-compose-file without-arg.yaml
# Use --kafka-container argument w/ non-default value and write output to `with-arg.yaml`:
pipenv run python3 kafka_docker_composer.py --shared-mode -b 3 -c 1 -s 0 -C 0 --kafka-container cp-kafka --docker-compose-file with-arg.yaml
diff without-arg.yaml with-arg.yaml

Expected:

The two docker-compose files should be using different images (cp-kafka rather than cp-server) for brokers and controllers and this should appear in the diff.

Actual:

The two docker-compose files are identical.