trusts: Make cn=adtrust agents sysaccount nestedgroup
    
    Since recent permissions work references this entry, we need to be
    able to have memberOf attributes created on this entry. Hence we
    need to include the nestedgroup objectclass.
    
    https://fedorahosted.org/freeipa/ticket/4433
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>