Title: #5927: ad: add required 'cn' attribute to subdomain object
If the forest root is not part of the return trusted domain objects
from the local domain controller we generate an object for further
processing. During this processing it is expected that the 'cn'
attribute is set and contains the name of the forest root. So far this
attribute was missing and it is now added by this patch.
To pull the PR as Git branch:
git remote add ghsssd https://github.com/SSSD/sssd
git fetch ghsssd pull/5927/head:pr5927
git checkout pr5927