Add layout validation env to tox.ini.

Change-Id: I7e3bd8eed585133ee217a1e462b9833df494be21
Reviewed-on: https://review.openstack.org/19718
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins