Add role templates path to jinja2 loader paths.

Lately, I've found myself wanting to include 'base' templates in a role, and then extend them with more specfic templates outside the roles. This makes the roles more reusable, and can be accomplished now by using a path like ```roles/nginx/templates/vhost.j2```. Where it starts to break down is when you have altered ```roles_path```, since now the nginx role might not be in the ```roles``` directory. So, in order to avoid coupling the path to the layout of your directory, I've made a small change to search the role's templates directory when using ```extends```.