Existing GSA name is not used as lookup for the service account

[johnraber]

https://github.com/terraform-google-modules/terraform-google-kubernetes-engine/blob/v16.1.0/modules/workload-identity/main.tf#L35.

Change account_id = var. name -> account_id = var. gcp_sa_name

data "google_service_account" "cluster_service_account" {
  count = var.use_existing_gcp_sa ? 1 : 0

  account_id = var. gcp_sa_name

  project    = var.project_id
}

[morgante]

Duplicate of #1002, which will be fixed in the next release. You can pin to master if you need a fix sooner.