Add BROKER_PORT and BROKER_HOST env vars.

Results in the following:

```
ubuntu@ip-10-178-32-130:/home/dokku$ dokku rabbitmq:link dev dev
-----> Linking rabbitmq_dev:broker with dev
-----> Setting config vars and restarting dev
BROKER_SCHEME:     amqp
BROKER_USERNAME:   admin
BROKER_PASSWORD:   xxxxxxxx
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}
-----> Releasing dev ...
export REDIS_URL=redis://172.17.0.3:6379

-----> dev linked to redis/dev container
-----> Injecting Shoreman ...
-----> Release complete!
-----> Deploying dev ...
-----> Deploy complete!

-----> dev linked to rabbitmq_dev container
```