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
diff --git a/tox.ini b/tox.ini
index adddba8..7a89a47 100644
--- a/tox.ini
+++ b/tox.ini
@@ -25,3 +25,6 @@
 
 [testenv:venv]
 commands = {posargs}
+
+[testenv:validate-layout]
+commands = zuul-server -c etc/zuul.conf-sample -t -l {posargs}