We have defined the DN_DM string constant for the Directory Manager:
https://pagure.io/lib389/blob/master/f/lib389/_constants.py#_68

but we don't always make use of it:
https://pagure.io/lib389/blob/master/f/cli/dsconf#_47
https://pagure.io/lib389/blob/master/f/cli/dsidm#_49

It's better to use the already defined constant instead of hard coding the value here and there.

I've made sure that there are no more occurrences in the source code by running:
grep -rnw ./lib389 -e "cn=Directory Manager"