This commit changes the trained model allocation task info
in two ways:

  - the action is now `xpack/ml/trained_model_allocation` and does
  not contain the model_id as this is consistent with other ES tasks.
  - the description is now `model_id[<model_id>]`. We do not need to
  prefix this with anything and it follows other ES tasks where information
  is added in square brackets following the information description.