I try doing use case with vitrage notification execute mistral but got a problem:

In my use case, i try evacuate host with 2 instance "vm_1" and "vm_2" on it, when host have alarm -then evacuate instance:
```

condition: nova_compute_down_on_host and host_contain_instance
actions:

  - action:
      action_type: execute_mistral
      properties:
         workflow: evacuate_vm
         input:
            vm_name: get_attr(instance, name)

```

I expect mistral workflow will then be: execute workflow 2 times with input:

```vm_name: vm_1```  and  ```vm_name: vm_2``` 

But i alway get : mistral execute that workflow 2 times with input:

```vm_name: vm_1```  and  ```vm_name: vm_1``` 

When i turn on debug log, to see the line "Function get_attr called with template_id..." follow
"https://github.com/openstack/vitrage/blob/stable/queens/vitrage/evaluator/template_functions/v2/functions.py"
I saw that line appear only one time!

I don't know it is bug or not? please help me
(my vitrage in queens version)

Best regards.