For everyday developer use, we don't need volumes for testing CSI. Providing a
flag to opt-in speeds up deploying dev clusters and slightly reduces infra costs.
Skip CSI test if missing volume specs.
This changeset adds volumes but does not mount them to instances so
that we can test the mounting ("staging") via CSI plugins. The CSI
plugins themselves will be installed as Nomad jobs.
In order to ensure we can always mount the EFS volume, this changeset
pins the deployment of the cluster to a specific subnet. In future
work we should spread the cluster out among several AZs and test that
behavior explicitly.
Tim Gross