IPA: use cache searches in get_groups_dns()
If the group name is overridden in the default view we have to search
for the name and cannot construct it because the extdom plugin will
return the overridden name but the DN of the related group object in the
cache will contain the original name.
Related to https://pagure.io/SSSD/sssd/issue/3579
Reviewed-by: Fabiano FidĂȘncio <fidencio@redhat.com>