ipaconfig: add the ability to manipulate domain resolution order
    
    optional attribute was added to config object along with validator that
    check for valid domain names and also checks whether the specified
    domains exist in FreeIPA or in trusted forests and, in case of trusted
    domains, are not disabled.
    
    Part of http://www.freeipa.org/page/V4/AD_User_Short_Names
    
    https://pagure.io/freeipa/issue/6372
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>