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. 
I suggest to do this and then you can delete `api/v2/compute_resources/associated_hosts.json.rabl`

```ruby 
      def associated_hosts
        @hosts = Host.for_vm(@compute_resource, @compute_resource.vms(:eager_loading => true))
        @total = @subtotal = @hosts.count
        render 'api/v2/hosts/index', :layout => 'api/v2/layouts/index_layout'
      end
```