XD-2475 MessageBus Producer Batching

JIRA: https://jira.spring.io/browse/XD-2475

Rabbit: Use a `BatchingRabbitTemplate` if batching is enabled.
Kafka: Set properties

Tested with:

    xd:>stream create foo --definition "time | log"
    xd:>stream deploy --name foo --properties "module.time.producer.batchingEnabled=true,module.time.producer.batchSize=6"

Observe messages appear in log in batches of 6.

    xd:>stream undeploy foo

Observe partial batch is flushed.

(Also added RabbitMessageBus test).
Actually, I missed the 'default' namespace in 'application.yml' :). My original comment was more targeted at the 'rabbit' references.