Move dask-gateway workers to their own nodegroups on the openscapes cluster

[yuvipanda]

Similar to #4482 but for dask-gateway.

• Create the same set of dask worker nodes for staging, prod and workshop
• Configure each of the hubs so their dask workers only spawn on the nodepools designated for them
• Tag each of the nodepool with 2i2c:hub-name to match the name of the hub
• Tag each of the dask nodepools with 2i2c:node-purpose set to worker

Definition of done

• Users across these hubs will spawn dask workers on to their own nodepools, and not share them with users on other hubs.
• There is a tag 2i2c:hub-name, 2i2c:node-purpose on nodes spawned by dask-gateway. You can verify this by looking at EC2 instances on the AWS console.