Allow node data bag path to be configured

Make it possible to configure the `node_path` that is used when nodes are searched. This is consistent with `data_bag_path` and `role_path`.
The default is still <data_bag_path>/node/.

This would help for example [knife-solo](http://matschaffer.github.com/knife-solo) as its node objects are by default in "nodes" directory that is sibling to "data_bags". See issue matschaffer/knife-solo#117.