New version of openstackdocstheme (Victoria+) respects pygments_style.
Since this repo is using now Victoria (master) requirements but has
not branched for Ussuri yet, it uses the new version.
Change pygments_style to 'native' since old theme version always used
'native' and the theme now respects the setting and using 'sphinx' can
lead to some strange rendering.
Change-Id: I97eed9ddd952b002830cae20136d6700c46901f9
5d8000967e