Fix dot-separated parentAssociationMapping

This fix allows to use dot-separated parentAssociationMapping identifiers. They now matche the filter form name (see https://github.com/sonata-project/SonataAdminBundle/blob/master/Filter/Filter.php#L58).

It's now possible to have, for example, a ``/users/{id}/comments`` with ``post.author`` as parentAssociationMapping in the CommentAdmin class.