Expose addr, port, proto env variables when linking, Closes #2430

Currently the {proto}://{ip}:{port} is exposed, but no variable with just the IP is exposed, it seems that often one will want just the IP of the linked container. 

For example in the redis documentation example, you might see the following in addition to the current environment variables:
```
# The ip of the service running in the container
DB_PORT_6379_TCP_ADDR=172.17.0.8
```