ipa-kdb: simplify trusted domain parent search

In terms of cross-forest trust parent domain is the root domain of
the forest because we only have trust established with the forest root.

In FreeIPA LDAP store all sub-domains stored in cn=<forest root>,
cn=ad,cn=trusts,... subtree. Thus, a first RDN after cn=ad is the
forest root domain. This allows us to simplify logic of finding
the parent domain.

For complex hierachical forests with more than two levels of
sub-domains, this will still be true because of the forest trust:
as forest trust is established to the forest root domain, any
communication to any sub-domain must traverse forest root domain's
domain controller.

Note that SSSD also generated incorrectly CA paths information
for forests with non-hierarchical tree-roots. In such cases
IPA KDC got confused and mistakenly assumed direct trust to the
non-hierarchical tree-root instead of going through the forest
root domain. See https://fedorahosted.org/sssd/ticket/3103 for
details.

Resolves: https://fedorahosted.org/freeipa/ticket/5738
Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>

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