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
Merge branch '4433_adtrust_agents_nested_group' into 4288_return_full_rootDSE