Enabled the use of extra vars in playbook file paths when including play...

...books from other playbooks.

How about this: when including playbooks from other playbooks, Ansible will now use extra-vars as well when generating the file path of the other playbook.

This allows, for example, delivering playbooks as part of Python packages, along with a simple wrapper script that can inject the package directory. Then I can instruct my users to, in their playbooks, simply: 

- include: {{my_app}}/my_playbook.yml.