oslosphinx theme is being retired as it is not compatible with the
recent Sphinx 2.0 release. This switches docs and release notes to use
the openstackdocstheme instead.
Change-Id: Ifc544d5d4ca9389a3a73d95397c6489fcab9d509
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>