The "security_group_api" config option in nova was deleted
in the Newton release:
I921650d8730201c2f14deb7e679647a892dbe48a
The use_neutron option should be used instead. This updates
the docs where the security_group_api option was mentioned.
Change-Id: Ie53a5498e1a2152157d5b2a56abb97ba36cbf86c