ad: handle forest root not listed in ad_enabled_domains
Although users and groups from the forest root should be ignored SSSD
will still try to get information about the forest topology from a DC
from the forest root. So even if the forest root domain is disabled we
should makes sure it is usable for those searches.
Resolves https://pagure.io/SSSD/sssd/issue/3361
Reviewed-by: Pavel Březina <pbrezina@redhat.com>