scheduler: volume updates should always be destructive

[tgross]

Fixes #12963

No task driver with file system isolation can support updating mounts in-place. Host volume and CSI volume or volume mount additions or updates should therefore always be destructive.

---

This changeset includes some unit testing but I've manually tested it on real CSI volumes as well. Adding a new volume:

$ nomad job plan ./demo/csi/hostpath/redis.nomad
+/- Job: "example"
+/- Task Group: "cache" (1 create/destroy update)
  +   Volume {
      + AccessMode:     "single-node-reader-only"
      + AttachmentMode: "file-system"
      + Name:           "volume0"
      + PerAlloc:       "true"
      + ReadOnly:       "true"
      + Source:         "test-volume"
      + Type:           "csi"
      }
  +/- Task: "redis" (forces in-place update)

Altering an existing volume:

$ nomad job plan ./demo/csi/hostpath/redis.nomad
+/- Job: "example"
+/- Task Group: "cache" (1 create/destroy update)
  +/- Volume {
    +/- AccessMode:     "single-node-reader-only" => "single-node-writer"
        AttachmentMode: "file-system"
        Name:           "volume0"
        PerAlloc:       "true"
    +/- ReadOnly:       "true" => "false"
        Source:         "test-volume"
        Type:           "csi"
      }
      Task: "redis"

[DerekStrickland]

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