Add generic split_any_principal method
There exist methods to split user or service/host principals, but
there is no method to split any kind of principal and allow the
caller to decide what to do.
Generalize ``ipalib.plugins.service.split_principal`` to return a
service of ``None`` if the principal is a user principal, rename it
``split_any_principal`` and reimplement ``split_principal`` to
preserve existing behaviour.
Part of: https://fedorahosted.org/freeipa/ticket/4938
Reviewed-By: Martin Basti <mbasti@redhat.com>