Make the host-service/backend & mandelboxes work for non-GPU instances

[philippemnoel]

Feature Description

Regarding the host-service, from @djsavvy:

I think it’ll require some trivial modifications to run on instances without GPUs. You just need to turn off gpu metrics collection.

Regarding the mandelboxes, from @philippemnoel:

I think we'll need to bring back the dummy virtual display, since we can't use the Nvidia one (I think) in this case. Maybe some conditional logic would be nice, since we should be able to deploy both non-GPU and GPU instances, and know which ones to have the host-service use at runtime automatically

Maybe the best approach is just to deploy and pre-download both on all instances unconditionally, and have the host-service have the logic for deciding which one to spin up based on whether it initializes GPUs or not.

[philippemnoel]

For the host-service, it might be as simple as following what we do for CI, which doesn't have GPUs! See:

		// CI does not have GPUs
		if !metadata.IsRunningInCI() {
			if err := mandelbox.AssignGPU(); err != nil {
				return utils.MakeError("Error assigning GPU: %s", err)
			}
		}