fixes #7336 - editing compute resource should not clear existing attributes from the form

@domcleal, I get the following error when I pass an invalid provider in the API. The excemption is caught in `def self.new_provider` rather than the validation on `@compute_resource`
https://github.com/theforeman/foreman/blob/develop/app/models/compute_resource.rb#L67-#L73

```
{
    "error": {
        "message": "ERF42-0410 [Foreman::Exception]: unknown provider"
    }
}
```