Feature Request: fall back to tests/dummy/config/deploy.js

I know it is possible to do:

```
// .ember-cli
{
  "ember-cli-deploy": {
    "deploy-config-file": "tests/dummy/config/deploy.js"
  }
}
```

but it would be great this was searched as fallback. This location makes more sense if you are an addon publishing your dummy app.