Make zone field consistent and truly optional for all resources

[rosbo]

#816 enabled the zone field to be specified at the provider level.

However, a few resources such as google_compute_disk are setting the zone field in the read method. This was causing a perpetual diff if the zone was inferred from the provider default zone:

-/+ google_compute_disk.my-disk[0] (new resource required)
      disk_encryption_key_sha256: "" => "<computed>"
      label_fingerprint:          "42WmSpB8rSM=" => "<computed>"
      name:                       "my-disk-0" => "my-disk-0"
      project:                    "rosbo-personal" => "<computed>"
      self_link:                  "https://www.googleapis.com/compute/v1/projects/REDACTED/zones/us-central1-c/disks/my-disk-0" => "<computed>"
      size:                       "100" => "<computed>"
      type:                       "pd-ssd" => "pd-ssd"
      users.#:                    "0" => "<computed>"
      zone:                       "us-central1-c" => "" (forces new resource)

This fix marks all the zone field as Computed and is consistent about always setting the zone field in the state inside the read method.

cc/ @ndmckinley

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

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