It is now possible to list users for sudo access.

templates/sudoers.d.j2 and defaults/main.yml:
- Added sudoers_users, which is an array that contains the users
that the sudoers can access to. Defaults to ALL to keep the actual
behaviour.

README.md:
- Decription of sudoers_users.

tasks/main.yml:
- Changed apt_cache_valid_time to sudoers_apt_cache_valid_time to
avoid collisions with other roles/playbooks.