rpk start: Allow passing --seeds and --advertised

[0x5d]

This will make it easier to run in general, but specifically in containers.

[0x5d]

Being able to pass it through an env var is a requirement too.

[dotnwat]

@0x5d we don't have to be able to pass through environment variables, but we need to be able to configure the advertised host configuration when starting up a docker container.

here is an example using environment variables, but it would be completely reasonable to take arguments via command that'll get appended to the entrypoint.

  kafka-broker:
    image: vectorized/redpanda
    ports:
      - 9092:9092
    expose:
      - 29092
    environment:
      KAFKA_AUTO_CREATE_TOPICS_ENABLE: "false"
      KAFKA_OFFSETS_TOPIC_REPLICATION_FACTOR: 1
      KAFKA_LISTENERS: PLAINTEXT://0.0.0.0:29092,OUTSIDE://0.0.0.0:9092
      KAFKA_ADVERTISED_LISTENERS: PLAINTEXT://kafka-broker:29092,OUTSIDE://localhost:9092
      KAFKA_LISTENER_SECURITY_PROTOCOL_MAP: PLAINTEXT:PLAINTEXT,OUTSIDE:PLAINTEXT
      KAFKA_INTER_BROKER_LISTENER_NAME: PLAINTEXT

[0x5d]

I've been wanting to port the rpk config mgmt to Viper for the longest time. That will make this way easier.