Create sudoers.d as a directory instead of a file

Before this commit /etc/sudoers.d was created as a file instead of a
directory if parameter sudoers was undefined. This prevents the module
from properly storing sudo rules in /etc/sudoers.d later when parameter
sudoers is defined.
Merge pull request #7 from anders-larsson/sudoers_d_directory_bug

Create sudoers.d as a directory instead of a file