Make the worker configuration optional

[adeelnawaz]

While configuring the bundle, a worker MUST be defined in queue configuration. It should be possible to use the bundle just for publishing messages without the worker.

Consider the use-case:
App A, B, and C populate the queue for a microservice app D. The apps A, B, and C do not need/have the worker; only the publisher. In order to use this bundle in publisher apps, they have to create a fake queue worker.