Explicit only glacier flexible storage class is supported for Cold Storage

Today, we consider that if the s3 storage class is not null, then it means a blob is under Glacier Flexible Retrieval storage class meaning under Cold Storage for Nuxeo. We should check that the storage class is indeed Glacier Flexible Retrieval as the Cold Storage addon has been designed to work with only this class.

For instance, some other classes such as Instant Retrieval do not require a Restore.