Creates flow arguments object for a flow with a given name.

def CreateFlowArgs(self, flow_name=None):
    """Creates flow arguments object for a flow with a given name."""
    if not self._flow_descriptors:
      self._flow_descriptors = {}

      result = self._context.SendRequest("ListFlowDescriptors", None)
      for item in result.items:
        self._flow_descriptors[item.name] = item

    try:
      flow_descriptor = self._flow_descriptors[flow_name]
    except KeyError:
      raise UnknownFlowName(flow_name)

    return utils.CopyProto(utils.UnpackAny(flow_descriptor.default_args))