Added "doctrine" alise to "doctrine_mongodb" service

We need this to have ability use:
```php
$this->getDoctrine()
```
in controllers that have extends by Symfony\Bundle\FrameworkBundle\Controller\Controller
There we have this:
```php
public function getDoctrine()
    {
        if (!$this->container->has('doctrine')) {
            throw new \LogicException('The DoctrineBundle is not registered in your application.');
        }

        return $this->container->get('doctrine');
    }
```