Using get_default_gcs_bucket_name to initialize the bucket

Similar to https://github.com/gae-init/gae-init/pull/97 we can initialize the `Config.bucket_name` with something from the `app_identity` (as found in the `google.appengine.api`).

See https://developers.google.com/appengine/docs/python/googlecloudstorageclient/activate?hl=nl#Using_the_default_Gcs_bucket which also explains that **new** applications, since GAE Python SDK v1.9.0, receive a default bucket (with a free quota; I believe of 5 GB). For existing apps, the link provides instructions on how to create the Google Cloud Storage (GCS) bucket by hand.