Change permissions on /etc/sudoers to be the same as standard build

The permissions on /etc/sudoers are 0440 on a standard build. This is the case for at least Ubuntu and CentOS which suggests that the permissions set here are incorrect. 

Having them changed to 0400 means they get flipped on every Puppet run (set correctly and then incorrectly by this module).

This pull requests sets the permissions to 0440 instead of 0400. Also includes updated test.