Improve the terraform for both backends

[waziers]

• remove un-used IAM permissions - we're using the minimum possible
• don't fail when the vertex auth token isn't available
• create the big query dataset in the main function instead of in the examples
• big query backend: don't create extra service accounts, we can just assign permissions to the service account created by the big query connection

[waziers]

@LukaFontanilla - i made the requested change. We are now creating the Service Account, and providing it with the roles as defined in https://cloud.google.com/looker/docs/db-config-google-bigquery#authentication_with_bigquery_service_accounts. I've updated the README as well for the additional instructions.