flatten and document get_returner_options().

Flattening the get_returner_options() func:
split into several funcs, return early when possible.

Document all arguments (it appears usage is quite a bit complex).

PEP8 almost everything.

(This code is ran very often in Salt. Hello Jenkins!)
Merge pull request #16907 from feth/refactor/returner_options_reading

rewriting returners/__init__.py : flatten, document, pep8