Fix node config attribute handling and make "nodes" directory path configurable

This PR should fix #247.

Use `node_path` to specify the directory where node configuration json files are stored. Default to "nodes" as earlier. It is worth noting that `node_path` defaults to "/var/chef/nodes" on Chef versions before 11.6.0. But as that directory should not exist, we will fallback to "nodes".

This also fixes the problem that when the json path was given as a command line argument to the cook command and the path was outside "nodes".

And as a bonus this sets `nodes_path` for future chef-solo-search versions.