Make rabbitmq host configurable

[rberlind]

This is excerpted from microservices-demo/microservices-demo#686

I'm running the Socks Shop apps on the Apcera platform in which I currently need to use fully qualified DNS names having the form .apcera.local rather than simple host names and ports different from 80 for some technical reasons which I would rather not go into here. This has caused me some problems including the following:

1. queue-master has the rabbitmq host hard-coded to "rabbitmq" in RabbitMqConfiguration.java and does not include the spring.rabbitmq.host setting in application.properties. Specifically, queue-master uses "CachingConnectionFactory connectionFactory = new CachingConnectionFactory("rabbitmq");". I would like to see queue-master changed to do what shipping does, namely add:
   @value("${spring.rabbitmq.host}")
   private String host;
   to RabbitMqConfiguration.java and add spring.rabbitmq.host=rabbitmq to application.properties. This would then allow me to add "--spring.rabbitmq.host=rabbitmq.apcera.local" to the queue-master start command as I did for shipping.

Thanks,
Roger Berlind