Add tripleo-image-elements forked from diskimage-builder.

Change-Id: I9bbccf79331ce45891ba6d07aa92c14a96543793
Reviewed-on: https://review.openstack.org/26134
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Anita Kuno <akuno@lavabit.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins