Make fetchSlice compatible with custom repo method

If a user configures query_builder_method for the doctrine provider, and does not alias the root entity to equal `Provider::ENTITY_ALIAS` then a DQL error will occur during index population.

```yaml
# config.yml
fos_elastica:
    # ...
    persistence:
        provider:
            query_builder_method: indexingQueryBuilder
```

```php
<?php

class AcmeRepository
{
    public function indexingQueryBuilder()
    {
        $qb = $this->createQueryBuilder('acme');

        $qb
            ->addSelect('categories')
            ->leftJoin('acme.categories', 'categories')
        ;

        return $qb;
    }
}
```

Causes this error:

> [Semantical Error] [...] near 'a.id ASC': Error: 'a' is not defined.

This is because FOS\ElasticaBundle\Doctrine\ORM\Provider adds an orderBy statement in fetchSlices, but the alias is hard coded to the constant `Provider::ENTITY_ALIAS`. This is a regression over previous versions of the bundle (introduced in b11b4299ef58d3c964a227f889527f2c484cc2c3 9 months ago).

This PR makes the alias dynamic in the same way that `Provider::countObjects` handles the alias.