csi: NodePublish should not create target_path, only its parent dir

[tgross]

Fixes #8358

The NodePublish workflow currently creates the target path and its parent
directory. However, the CSI specification says that the CO shall ensure the
parent directory of the target path exists, and that the SP shall place the
block device or mounted directory at the target path. Much of our testing has
been with CSI plugins that are more forgiving, but our behavior breaks
spec-compliant CSI plugins.

This changeset ensures we only create the parent directory.

[tgross]

Edit: fixed in e5bdcda

This seems to be "working" but has an unpleasant ergonomic result that also breaks backwards compat with existing jobspecs. With a volume mount like:

volume_mount {
  volume      = "test"
  destination = "${NOMAD_TASK_DIR}/test"
  read_only   = false
}

We end up with the mount showing up in the task at /local/test/rw-file-system-single-node-writer. So I need to correct how we pass the final binding into the container.

[langmartin]

looks fine

[tgross]

I've tested on our usual set of regression targets as well as the DO plugin from the original report in #8358. Going to merge this and it'll ship in 0.12.2

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