Add new wm helper list_objects_json to chef_wm_base

This helper uses chef_db:list to fetch object names and generate the
JSON mapping of name to URL in a way that can be reused for most
object types.

The helper behaves like a to_json/2 and you can use it by importing it
into a wm resource module via mixer and renaming it to_json.
Use list_objects_json in chef_wm_roles