Respect outputPaths in ember-cli-build

I recently implemented this addon and noticed that the index.json output does not respect the `outputPaths` config setting in `ember-cli-build.js`. Here is a sample of my configuration and the resulting index.json output from `ember deploy production`:

```
// ember-cli-build.js
const isProduction = ( process.env.EMBER_ENV || 'development' ) === 'production';
const assetPath = isProduction ? '' : '/assets';

module.exports = function(defaults) {
var app = new EmberApp(defaults, {
outputPaths: {
app: {
html: 'index.html',
css: {
'app': assetPath + '/app.css'
},
js: assetPath + '/app.js'
},
vendor: {
css: assetPath + '/vendor.css',
js: assetPath + '/vendor.js'
}
}
});
}
```

```
// config/deploy.js
var ENV = {
build: {},
'json-config': {
jsonBlueprint(context, pluginHelper) {
var jsonBlueprint = pluginHelper.readConfigDefault('jsonBlueprint');
jsonBlueprint.script.includeContent = true;
return jsonBlueprint;
}
}
};
```

```
// index.json
{
"base": [],
"meta": [],
"link": [{
"rel": "stylesheet",
"href": "assets/vendor-13455b8fd285c046141dd54ae73890f8.css"
}, {
"rel": "stylesheet",
"href": "assets/app-efe517a8fe179e4392e64c2a9ce8fe05.css"
}],
"script": [{
"src": "assets/vendor-127d2a9b59896b5ddff4de18193179cb.js"
}, {
"src": "assets/app-774827c1c48acfbdb034cbc16d3f5fb7.js"
}]
}
```

Start with the plugin code that builds index.json and compare its generated asset paths with the outputPaths configuration shown in ember-cli-build.js. Use config/deploy.js and the sample index.json as references, then run ember deploy production to verify that the generated link and script paths honor the configured outputPaths.

Written by the indexing model from the issue text.