create node settlement extension

This extension can be used to decide where to attach a newly created
node.
You could restructure this code to make it more readable:

````
$dm = $admin->getModelManager()->getDocumentManager();
$path = $this->nodeSettlementStrategy->getParentNodePath($object);
$node = $dm->find(null, $path);

if (!$node) {
    throw new Exception(...);
}

$object->setParentDocument($documentRoot);
````