A Starlingx patch[1] changed the signature for some openstackdocstheme
methods which is causing Octavia docs to not build.
This patch updates the octavia docs configuration for the new
openstackdocstheme.
[1] https://review.openstack.org/#/c/607298
Change-Id: Id09ab3b78291c28e116f1f4ffb8836eac0537d94