Detect s3 object-storage service

[garloff]

In the self-description, we can easily list swift object storage, but we can not easily discover S3.
I fear we would need to really probe, i.e. generate an EC2 credential and check whether we can talk to the swift endpoint via the s3 protocol. Doing so, we could also detect whether swift and s3 are referring to the same storage. Of course care needs to be taken to clean up again after discovery.