ipa-client-install: Use direct RPC instead of api.Command
    
    To make sure the installation works with older servers,
    use XML-RPC directly, with a version set explicitly so the request
    is not rejected.
    
    RPC was chosen over ldapmodify, because going through the API allows
    the server to process the request properly, or even cleanly reject
    it if there are incompatible changes in future versions.
    
    https://fedorahosted.org/freeipa/ticket/3931