There are 3 layers of variable definations:
 group_vars/securedrop.yml
 host_vars/{app.yml mon.yml}
 (env)-specific.yml

The master playbook 'site.yml' includes all of the playbooks except the travis playbook.
This is because the travis playbook's host is 'localhost' which would mean that any host that runs the site.yml will run the travis playbook on their host sytem