Per this email[0], it's time to migrate our API docs into our tree.
This copies the sphinx extension from nova into our tree as well, which
will be later split into its own python module. At that time we can cut
over to the module and drop it from our tree.
The conf.py and the tox environment are also cribbed from nova.
[0] http://lists.openstack.org/pipermail/openstack-dev/2016-May/093765.html
Change-Id: I683cc4d6459d77c42951a06247b6f9ff8f8456bd
fa098f69e4