Add support for the model_class attribute & fix compiler pass argument error

[nocive]

Subject

I am targeting this branch, because it's a backwards compatible change.

Fixes

In Definition.php line 321:
                                                                                               
  [Symfony\Component\DependencyInjection\Exception\OutOfBoundsException]                       
  The argument "1" doesn't exist in class "My\Sonata\AdminClass".  
                                                                                               

Exception trace:
  at /src/application/core/vendor/symfony/dependency-injection/Definition.php:321
 Symfony\Component\DependencyInjection\Definition->getArgument() at /src/application/core/vendor/sonata-project/doctrine-orm-admin-bundle/src/DependencyInjection/Compiler/AddAuditEntityCompilerPass.php:52
 Sonata\DoctrineORMAdminBundle\DependencyInjection\Compiler\AddAuditEntityCompilerPass->process() at /src/application/core/vendor/symfony/dependency-injection/Compiler/Compiler.php:82

When using the model_class tag attribute, rather than using explicit indexed arguments.

Changelog

### Fixed
- Support for the `model_class` attribute for sonata admin classes in `AddAuditEntityCompilerPass`.

[nocive]

cc @VincentLanglet