Resolve "Autopilot should use provided service account rather than always using default SA"

Tested with custom sa:

gcloud container clusters describe cluster --region=europe-west2
...
autopilot:
  enabled: true
...
nodeConfig:
...
  serviceAccount: test-sa-for-gke-autopilot@gitlab-test-caafe422.iam.gserviceaccount.com
...

Closes #43 (closed)