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: I93ed87772eeea389ebf6d1cb95241091d54e9ae0
f73cc34a51