Add a configuration key so no new files go to remote storage

In gitlab-ce!31502 @mkozono brought up an important point:

When migrating from local to object storage, the flow is:

• Configure and enable object storage
• Now all new files are going to object storage
• Migrate existing files

Going backwards, it seems like we need to add configuration that allows object storage to be configured and "enabled" for existing files, but "new files go to local storage". Please correct me if I'm missing something.

• Existing files are in object storage
• Change configuration so new files go to local storage
• Migrate existing files
• Disable object storage

This would enable users to migrate from remote storage to local storage without requiring downtime.