Update gcs settings to override the new credentials_source default
Update gcs settings to override the new credentials_source default

- When credentials_source, the new default is to make it read only (https://github.com/cloudfoundry/bosh-gcscli/commit/464c3ab83d79201071318a641f4fec5498fdc901), for public gcs buckets.  Use static to make it read/write.

[#152560267](https://www.pivotaltracker.com/story/show/152560267)
cloudfoundry/docs-bosh