[Tree] Retrieve querybuilder with getQueryBuilder from AbstractTreeRepository

Problem : each method who need a query builder retrieve it by the following `$this->_em->createQueryBuilder()`. 
With this way we can't inject our own QueryBuilder (e.g: new PageQueryBuilder($options) to retrieve
a prepopulated QueryBuilder according to entity business (left join, or all what we want)

Use ` getQueryBuilder`  method to retrieve it make this possible, we just
need to overload her in this case.