fixes #4155 - associate and disassociate vms using the api

I'm not sure I see a how to associate individual vm's, but this would allow disassociations and mass associations for compute resources. 
same here, you should be able to call

```ruby 
        render 'api/v2/hosts/index', :layout => 'api/v2/layouts/index_layout'
```
and remove the `associate.json.rabl` template