Return ports exposed by the image

Return a list of ports exposed by the base image alongside the user-exposed ports on a service. We want to differentiate between these to preserve the template-defined state.
```
{
    ...
    "expose": [],
    "default_exposed_ports": [
        "6379/tcp"
    ],
    ...
}
```
How about
```
result.present? ? result.keys : []
```

The `.present?` bit probably isn't even really needed -- from what I can tell the `ExposedPorts` are either there and have at least one key, or it isn't present at all. The `.present?` would protect us in the event they ever returned something like `'ExposedPorts': {}`.