Add audit 'force' option

This PR adds a ``force`` option to audit (``true`` by default to keep BC).

```
    sonata_doctrine_orm_admin:
        audit:
            force: true
```

If ``force`` is ``true``, all entities will be audited by default, unless ``audit="false"`` is set (previous default behavior).
If ``force`` is ``false``, entites won't be audited by default, unless ``audit="true"`` is set.

I use a project where I want to audit only 1 entity, and that's much easier with this option.