Backport of 'Retry GCS default credentials after a latched metadata timeout'

What does this MR do and why?

Backport of !266794 (merged) to 19.5.

With GCS object storage and google_application_default: true, a single GCE metadata-server timeout makes google-cloud-env cache "not on GCE" for the life of the process. Every later Fog::Storage.new then fails with Your credentials were not found until the process restarts. The fix adds an initializer that resets the latched credentials cache and retries once when this happens.

The cherry-pick was clean. fog-google, googleauth and google-cloud-env are at the same versions on this branch as on master.

Related to #636314 (closed)

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