tf-modules/gcp/gke: ignore node_config changes

[darkowlzz]

When using workload identity in GKE, an empty node_config.oauth_scopes is set at provision time. But after the provision, some default scopes are added to the cluster. Reapplying the same config tries to replace the existing scopes with empty scopes. This replacement requires the existing cluster to be deleted and recreated. To avoid cluster recreation between test runs, set a lifecycle rule to ignore any changes in node_config.

Encountered this while testing fluxcd/pkg#665 . Was observing the following when rerunning the tests:

  # module.gke.google_container_cluster.primary must be replaced
-/+ resource "google_container_cluster" "primary" {
      ...

          ~ oauth_scopes      = [ # forces replacement
              - "https://www.googleapis.com/auth/devstorage.read_only",
              - "https://www.googleapis.com/auth/logging.write",
              - "https://www.googleapis.com/auth/monitoring",
              - "https://www.googleapis.com/auth/service.management.readonly",
              - "https://www.googleapis.com/auth/servicecontrol",
              - "https://www.googleapis.com/auth/trace.append",
              + "https://www.googleapis.com/auth/cloud-platform",
            ]
      ...

[darkowlzz]

After thinking more about the problem and going through the docs, the core issue can be avoided by using null instead of [] when there's a need to not pass any value. Refer fluxcd/pkg#665 (comment).
This change is no longer needed.