optional_config method and DAG config param check

[briangrahamww]

We should add the ability to specify an optional method for optional_config, which returns a set of config params similar to the necessary_config. Then we could have the sphinx autodoc gen tools properly pick up and format the optional config params from the docstring.

We could then add a check after necessary config to make sure and params in the DAG node config are in the set of the necessary config and optional config params, and return an error noting which params that were specified in the DAG config file are not used by the node.