fixes #6861 - provide a way to detect host group provisioning

Actually the method redirection in Host is even easier to see this way :)

```
[4] pry(main)> Host.first.is_a?(Host)
  Host::Managed Load (0.7ms)  SELECT "hosts".* FROM "hosts" WHERE "hosts"."type" IN ('Host::Managed') LIMIT 1
=> false
```