New version of openstackdocstheme (Victoria+) respects pygments_style.
Since openstack-ansible starts using 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: Ia8c05e4791a3db5eaa82593990d378d2d091c820
4767a92cd4