Moves Nova v3 API enable config to group

https://review.openstack.org/#/c/29487/ moves the Nova v3 API
enable flag into its own config group. And renames it to just
"enable". This changeset follows that change. As we do not yet
gate on any Nova V3 API tests we don't need to keep the old setting
around.

Change-Id: If1592cd7b9aad24b6d010870a5fb6bcc60efb8ac
Merge "Moves Nova v3 API enable config to group"