Docs: Fix conf.py for readthedocs.org

Additional files (subtests.rst and defaults.rst) are produced
for documentation by statically examining other project files.
Normally this would be accomplished by a Sphinx extension, however
for our simple needs, it's easier to use the existing method
employed in the Makefile.

Signed-off-by: Chris Evich <cevich@redhat.com>
Merge pull request #397 from cevich/fix_readthedocs

Docs: Fix conf.py for readthedocs.org