Use gcloud sdk/cli local authentication (instead of service-account)

[frivoire]

On our machines, we already have access with gcloud.
So the python script could use it directly (no config), and thus not requiring to pass a service-account that we usually don't have on a local machine (because it's not a "service")

cf: Google's recommendations: https://googleapis.dev/python/google-api-core/latest/auth.html#overview

If you’re developing locally, the easiest way to authenticate is using the Google Cloud SDK: