Update node-pool creation command
Update node-pool creation command

- We now use Container Optimized OS instead of Ubuntu for
  base images. This is far more locked down & secure than
  the Ubuntu base image, but used to not support NFS mounts.
  It now does, so we can switch to it - see #2024
- We use Balanced SSDs for all disk base images, rather than
  SSDs. This is cheaper (0.1 per GB rather than 0.17 per GB),
  and should hopefully provide enough image pulling performance
  for us. If not, we can always move back.
- I've also moved the alpha & beta pool to use the newer configuration.
  I'll move gamma in a day or two if things go well.

Closes #2024

ryanlovett/datahub,berkeley-dsep-infra/datahub,ryanlovett/datahub,berkeley-dsep-infra/datahub,berkeley-dsep-infra/datahub,ryanlovett/datahub