Improve Error Message with Reservation Validation

[arajmane-g]

About the Change

We already have validation on the reservation usage in GKE clusters. However, that error message can improved.

With this PR, the error message provides more information about which properties mismatch, the values configured on the reservation and the node pool, the relevant node pool settings, their defaults. So that, the user knows what is wrong and they can make corrections to the blueprint accordingly.

This PR, also, adds README instructions about using the reservations.

Tests

Manually tested different scenarios:

• no reservation: terraform plan operation succeeds
• any reservation: terraform plan operation succeeds
• valid specific reservation: terraform plan operation succeeds
• specific reservation with mismatches: terraform plan fails as expected; the error message includes the messages about the properties that mismatched, the values configured on the reservation and the node pool, the relevant node pool settings, their defaults.

For a concrete example, consider a scenario where the reservation has 1 nvidia-l4 accelerator, 0 disks and the machine type is g2-standard-4. Whereas, the node pool config from the blueprint is as shown below

  - id: g2_pool
    source: modules/compute/gke-node-pool
    use: [gke_cluster, gke_service_account]
    settings:
      machine_type: g2-standard-24
      local_ssd_count_nvme_block: 16
      reservation_affinity:
        consume_reservation_type: SPECIFIC_RESERVATION
        specific_reservations:
        - name: specific-reservation-1

The error message will look like the following as all of the machine type, local ssds and guest accelerators settings mismatch.

Check if your reservation is configured correctly:
- A reservation with the name must exist in the specified project and one of
the specified zones

- Its consumption type must be "specific"

- The reservation has {"nvidia-l4":1} accelerators and the node pool has
{"nvidia-l4":2}. Check the relevant node pool setting: "guest_accelerator".
When unspecified, for the machine_type=g2-standard-24, the default is
guest_accelerator=[{"count":2,"type":"nvidia-l4"}].


- The reservation has {"NVME":0,"SCSI":0} local SSDs and the node pool has
{"NVME":16,"SCSI":0}. Check the relevant node pool settings:
{local_ssd_count_ephemeral_storage, local_ssd_count_nvme_block}. When
unspecified, for the machine_type=g2-standard-24 the defaults are:
{local_ssd_count_ephemeral_storage=0, local_ssd_count_nvme_block=0}.


- The reservation has "g2-standard-4" machine type and the node pool has
"g2-standard-24". Check the relevant node pool setting: "machine_type"

Submission Checklist

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• Test all changes with pre-commit in a local branch #
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• Add or modify unit tests to cover code changes
• Ensure that unit test coverage remains above 80%
• Update all applicable documentation
• Follow Cluster Toolkit Contribution guidelines #