Would be better with --log-opt instead of environment variables

[artheus]

It would be a preferred way to handle options for a log driver with --log-opt, since this is how every native (to docker) Log driver is designed.

From your examples it seems that we right now must set the environment variable for the Kafka Brokers when we install this plugin. I would really prefer the setting to be done on a per-container level. What if I really want separate brokers depending on the container, or different topics per container?