As per:
http://lists.openstack.org/pipermail/openstack-dev/2015-August/073338.html
The point behind the addition of the stevedore.sphinxext extension is
"to document drivers and other types of plugins to make the available
sets built into projects easier to discover" (dhellman).
Change-Id: I8427398e8d01c36f346e493a8144c1012ca7c5bf
