ipasam: use more restrictive search filter for group lookup
Since we are interested in looking up the SID of a group it makes sense
to include the objectclass which contains the SID attribute in the
search filter. This makes sure the group is not accidentally found a
second time in the compat tree.
Related to https://fedorahosted.org/freeipa/ticket/5457
Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>