Split the EBS and EFS tests out into their own test cases:
* EBS exercises the Controller RPCs, including the create/snapshot workflow.
* EFS exercises only the Node RPCs, and assumes we have an existing volume
that gets registered, rather than created.
Add a `PerAlloc` field to volume requests that directs the scheduler to test
feasibility for volumes with a source ID that includes the allocation index
suffix (ex. `[0]`), rather than the exact source ID.
Read the `PerAlloc` field when making the volume claim at the client to
determine if the allocation index suffix (ex. `[0]`) should be added to the
volume source ID.
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.
