Add yamllint check to tox validate environment
Add yamllint check to tox validate environment

This will help to catch issues in yaml sytax for rdo.yml and
deps.yml as duplicate keys in a dict.

Change-Id: Ibbb9b34b3578334a3a30cba617ef1eca89e22a5f

javierpena/rdoinfo,redhat-openstack/rdoinfo