Automatically handle subunit v1 or v2 streams.

Change-Id: Ib1ef05a6aa66606d03eca81d50ba84f6e6f41c1b
Reviewed-on: https://review.openstack.org/26688
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins