Allow Cloud Storage usage without S3 features

[agritheory]

There's a compelling case to be made that the File preview features of Cloud Storage are good even without the remote file storage. To ensure that this feature works correctly, it would be appropriate that we have a test that all these features work without setting any of the frappe.conf settings.

[Alchez]

@agritheory I think I'll need some help for the pytest configuration to start testing this one. Let's try and do a call today?