Debian 9 VMs currently broken; don't automatically load GCE credentials for standalone gsutil installations

This is due to /etc/boto.cfg being missing on Debian 9; as far as I can tell, this started within the past couple of weeks.

For more details, see the issue I filed against GoogleCloudPlatform/google_compute_engine: https://github.com/GoogleCloudPlatform/compute-image-packages/pull/820

Start by reviewing how standalone gsutil installations load credentials when /etc/boto.cfg is absent, with Debian 9 as the reproduction environment. Read the referenced compute-image-packages pull request for the related change. Done means standalone installations no longer automatically load GCE credentials while normal credential configuration continues to work.

Written by the indexing model from the issue text.