Use oslo.sphinx and remove local copy of doc theme

Use the new oslo.sphinx version of the OpenStack doc
theme instead of copying it into this repo.

blueprint oslo.sphinx

Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>
Change-Id: I0bd91f7bb43f97b99051fed65b75fc05d5149cc8
Merge "Use oslo.sphinx and remove local copy of doc theme"