rpcserver: assume version 1 for unversioned command calls
    
    When a command is called on the server over RPC without its version
    specified, assume version 1 instead of the highest known version.
    
    This ensures backward compatibility with old clients, which do not support
    versioned commands and understand only the first version of any given
    command.
    
    https://fedorahosted.org/freeipa/ticket/6217
    
    Reviewed-By: David Kupka <dkupka@redhat.com>