Simplify python service accounts on app engine

[sqrrrl]

Code for async-bot currently requires downloading a key and including it in the app. Recommended practice is to use environment-provided service accounts when deployed to GCP as this minimizes exposure (zero exposure) of the service account key.

Should switch to use google.auth.default() and only recommend downloading keys for local development or deployment to other clouds.

Pending merge of googleapis/google-auth-library-python#376 and release of new version of auth library.