Create separate option discovery functions for each piece of middleware,
so an application that only uses one piece can include only those
options in its sample configuration file.
Test the results with some unit tests that simply ensure the functions
do not raise exceptions, and by adding configuration options to the
documentation for the library, using the new integration of oslo.config
with sphinx.
Change-Id: I4c777cd70c063441f430c48ab1f9c9cac2c1fc75
ea893e8ba4