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'm having second thoughts about this - what's it trying to do?  Isn't this just an expensive (due to `.vms`) equivalent to doing `GET /api/hosts?search=compute_resource_id=1` ?  I can see a little use if it was essentially a no-op version of the `PUT /api/compute_resources/:id/associate` or perhaps simplify it to `Host.where(:compute_resource_id => @compute_resource.id)`, but not sure if that's worthwhile.