Ticket 20 - Use the DN_DM constant instead of hard coding its value
Bug Description: We have defined the DN_DM string constant but we hard code
its value in a few places instead of using it.
Fix Description: Replace the hard coded occurrences with the DN_DM constant.
https://pagure.io/lib389/issue/20
Author: Ilias95
Review by: wibrown (Thanks for your patch!)