Follow the new PTI for document build

This review already follows the new PTI
https://review.openstack.org/#/c/530875/

However, we also should remove [build_sphinx]
section as described in:
http://lists.openstack.org/pipermail/openstack-dev/2018-March/128594.html

Change-Id: Ifbfe80024731d2c552a5aab90e025f9ebd593c4d
1 file changed