Drop the default secret name for gcs object storage

The default value was for already more appropriate for s3, as it was called 's3access'. But the use of a default was previously dropped for s3 to allow iam roles. This change also drops the default for gcs.

Depends on !166 (merged) being merged first

This is a breaking change, so it being considered for 1.0 #89 (closed)

closes #141