Refactors the rocoto task generation to be recursive. This will allow
nested metatasks to loop over multiple variables, which is needed for
GEFS product generation.

As part of this refactor, there is no longer separate arguments to
designate metatasks. Instead, task dicts can include a nested 'task_dict'
as well as a 'var_dict' containing the variables to loop over. The nested
task dict can then either have another layer, or be the innermost task.

To accomodate the new recursive nature, some defaults that were previously
defined in `create_wf_task()` had to be pushed down into the function that
creates the innermost task. Also, former keywords have been absorbed by
the task dict.

Refs NOAA-EMC#823
Refs NOAA-EMC#827

Returns `rocoto.py` to a workflow-agnostic state without needing
`cdump` or any rocoto entities. The middle-man `create_wf_task` in
task.py is also removed by explicitly setting several keys in the
task dict.