Use milestone-proposed branch for stable versions

Change-Id: I497fe676966d2be655305fb3485ccefe09a8cb31
Reviewed-on: https://review.openstack.org/34184
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins