rose.config: support optional opt conf keys
Document optional opt conf keys
Merge pull request #1397 from matthewrmshin/optional-opt-conf-key

rose.config: support optional opt conf keys
rose.config_tree: improve pep8 and pylint scores
rose.config_tree: store file lists

If a relative path can be found in 2 or more locations in the linearised
inheritance, it will store the configuration locations in a new list.
Use rose.config_tree to load metadata

Part 1.
rose.macros.trigger: use rose.config_tree

to load metadata.