Bug 1062829 - Converted Json methods from XHR to LazyLoader in FTU APP

https://bugzilla.mozilla.org/show_bug.cgi?id=1062829
You don't need this promise anymore since the ``.then()`` method returns a promise too, so this code can be changed from:
```javascript
this._configPromise = new Promise(function(resolve, reject) {
  // ...
}
```
... to:
```javascript
this._configPromise = LazyLoader.getJSON(configURL).then(function(resolve, reject) {
 // ...
});
```
Also the ``_configRequest`` field isn't used anymore so you should remove it and its uses from the file. Check that the tests still work afterwards as its only use was in the ``reset()`` function that was used for tests.