Issue: Parent is missing in the fields list that causes RoutingMissingEx...

...ception on flushing

Having config:

```
                extra_field:
                    mappings:
                        text: ~
                        title: ~
                    _parent: { type: "user", property: "owner", identifier: "id" }
                    persistence:
                        driver: orm
                        model: Foo\CoreBundle\Entity\ExtraInfo
                        provider: ~
                        listener: ~
                        finder: ~
```

When building the project getting an exception on fixture loading when flushing ExtraInfo: 
```                                                                                      
  [Elastica\Exception\ResponseException]                                                
  RoutingMissingException[routing is required for [foo_dev]/[extra_field]/[1]] 
```

The  problem was that ObjectPersister got only the entity fields without the _parent field.
The reason was that the service in the ProjectContainer was generated incorrectly.

I'm not sure it's the best way to fix, please correct me if I'm wrong.