Enable Keystone extensions for OSC functional jobs

Devstack already has code to setup extensions for Keystone;
OpenStackClient has command support for these extensions.
Let's enable these extensions in DevStack before firing it up,
so we can add functional tests for them.

Change-Id: I8833c2421ff28454134854dd8570743f5b33248e
Merge "Enable Keystone extensions for OSC functional jobs"