factor out `populate_remote_domain` method into module-level function
    
    This allows for re-use of this method in cases where the caller can not or
    wishes not to instantiate local Samba domain to retrieve information about
    remote ones.
    
    https://fedorahosted.org/freeipa/ticket/6057
    
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>