Align Sphinx autodoc options with oslo.

The Sphinx autodoc changes also went into olso a little differently.

Change-Id: If1f337db693053f2627c0c426a51bc13128033a7
Reviewed-on: https://review.openstack.org/25676
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Monty Taylor <mordred@inaugust.com>
Tested-by: Jenkins