RabbitMQ: connection refused

[Louson]

Related to #1224

RabbitMQ is said to be optional. If so, it should not print an error.

Steps to reproduce:

• run the docker command: docker run -d -p 8080:8080 hawkbit/hawkbit-update-server
• the error is repeated infinitely

[easybe]

Setting spring.cloud.bus.enabled to false seems to do the trick.