Make OrchestrationService retirement work again

With this commit we make sure retirement request validation
succeeds, because right now it's failing with:

```
[ActiveRecord::RecordInvalid]: Validation failed: OrchestrationStackRetireTask:
    Request type should be orchestration_stack_retire
```

Reason for the error above ^ is that `OrchestrationStack.class.name` gets
transformed into `orchestrationstack` while validation expects underscore
inbetween: `orchestration_stack`.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1632239

Signed-off-by: Miha Pleško <[email protected]>

app/models/service_retire_task.rb

@@ -58,7 +58,7 @@ def create_task(svc_rsc, parent_service, nh, parent_task)
         :parent_service_id   => parent_service.id,
         :parent_task_id      => parent_task.id,
       )
-      task.request_type = svc_rsc.resource.type.demodulize.downcase + "_retire"
+      task.request_type = svc_rsc.resource.type.demodulize.underscore.downcase + "_retire"
       task.source = svc_rsc.resource
       parent_task.miq_request_tasks << task
       task.save!