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

I don't think this will work due to STI - Host is a fake class. I think you want Host::Base

```
[2] pry(main)> Host::Managed.new.is_a?(Host)
=> false
[3] pry(main)> Host::Managed.new.is_a?(Host::Base)
=> true
```