Expose {NAME}_IP env variable 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. I would implement this by exposing the information as `{NAME}_IP`.

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_IP=172.17.0.8
```