Added USER environment variable to start rabbimq with a preconficured username

Hi

We added a USER environment variable enabling us to start docker with a specific username (not "admin"). Please merge because we would really like to use your rabbitmq docker in the future :)

example:
docker run -d -p 5672:5672 -p 15672:15672 -e RABBITMQ_USER="guest" -e RABBITMQ_PASS="mypass" tutum/rabbitmq