Enabling Dynamic port Assignment for Container

We wanted to be able to assign ports for the rabbit container to listen on, so that multiple rabbitmq containers could be run on the same machine. We used the rabbitmq.config file properties and the start script to enable this.
We kept backward compatibility so that existing uses of the image will not be broken in any way.
Would love your review and feedback.
@pilwon would appreciate your comments, even if this isn't good for you, so we know to move forward and fork...