Add option to `create-notebook` to use GPUs

### Why it needs to get done

Based on the [dss spec](https://docs.google.com/document/d/1v0wU8TBb-ui6RcjGRbLwA6XFZnWbuiVb2TPmEgwKejY/edit), we need notebooks that support GPUs. #31 implements the `create-notebook` command, but it does not include an option to enable a GPU.

### What needs to get done

1. add an interface to `create-notebook` to enable gpus (`--gpu` flag? or something more detailed?)

### When is the task considered done

1. `create-notebook` allows creation of notebooks with GPUs