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: I3643bb9d00bb4109ec133e072b889a72f5a3248e