IPA: use ipaUserGroup object class for groups
dfb34c6c82ed5014599bf70de6791e6d79106fc2 changed object class
of IPA groups from posixGroups to more general groupOfNames.
However, this object class is used also for roles, permissions and
privileges which caused SSSD to consider those objects to be groups as
well during initgroups.
Reviewed-by: Jakub Hrozek <email@example.com>