Add BROKER_URL.

BROKER_URL finally works just the way I want it :)
Also maintains backwards compatibility.

```
ubuntu@ip-10-178-32-130$ dokku rabbitmq:link test-www test-www
-----> Linking rabbitmq_test-www:broker with test-www
-----> Setting config vars and restarting test-www
BROKER_SCHEME:     amqp
BROKER_USERNAME:   admin
BROKER_PASSWORD:   RFqnOe8vPHpUbWFn
BROKER_PATH:       //
BROKER_HOST:       ${BROKER_PORT_5672_TCP_ADDR}
BROKER_AMQP_PORT:  ${BROKER_PORT_5672_TCP_PORT}
BROKER_ADMIN_PORT: ${BROKER_PORT_15672_TCP_PORT}
BROKER_URL:        amqp://${BROKER_USERNAME}:${BROKER_PASSWORD}@${BROKER_PORT_5672_TCP_ADDR}:${BROKER_PORT_15672_TCP_PORT}//
-----> Releasing test-www ...
export REDIS_URL=redis://172.17.0.5:6379

-----> test-www linked to redis/test-www container
-----> Injecting Shoreman ...
-----> Release complete!
-----> Deploying test-www ...
-----> Deploy complete!

-----> test-www linked to rabbitmq_test-www container
```