Allow service-specific bind_addresses

While one can specify bind_address in the haproxy section of the config, this applies to all frontends globally. I would like to be able to override this per frontend, by specifying it with the port in the haproxy section of my service thus:

```
      "haproxy": {
        "bind_address": "1.2.3.4",
        "port": "80",
        ....
```