Move /etc/sudoers from module "admin" to "sudo"

This isn't specific to the admin module, so might just as well move it
to the sudo module and include "sudo" instead.

The sudo module also had a "sudoers.labs" sudoers file that would
conflict with this, but fortunately it had the exact same contents, so
we can just drop it. Yay deduplication!

This is part of a broader admin/sudo consolidation.

Change-Id: I43e0c56c055c0f213b49a1f59ce1f049d86c95b8