automatically include all variables in roles/foo/vars/

This patch implements RFE 2958. Additional vars/ files in the
roles/foo/vars/ will be automatically added.

To test you can simply rename any roles/foo/vars/main.yml into roles/foo/vars/a.yml and b.yml and observe that the vars are still available for the role.

I'm happy to 
- cleanup/improve of the code in any way required
- provide a test project for easy manual testing
- provide a proper integreation test 
if there is a chance that this feature gets in :)

I find it quite useful and natural to organize variables this way.

Thanks for your consideration,
 Michael