Before, the script was using OSH_EXTRA_HELM_ARGS to configure
virt_type and cpu_mode. This unfortunately overrode any passed in
OSH_EXTRA_HELM_ARGS.
Now, use OSH_EXTRA_HELM_VIRT_ARGS so users can still specify
OSH_EXTRA_HELM_ARGS when using `openstack.sh`.
Change-Id: I34019f2f751bd2e1bbbfdc0a17b99613e478369c