Add layout file validation.

Based on voluptuous library.

Basic validation should catch typos, missing or extra attributes.
Can be expanded to do more serious validation (ie, specifying
a comment in a trigger should require the event be comment-added).

Adds a command line option to validate a named layout file and
exit.

(Also add dist/ to .gitignore.)

Change-Id: Ia864ebde1765141d4d1a52bc77033689b6210e81
Reviewed-on: https://review.openstack.org/19443
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins