Bucket prefixes for S3

[Alessandro Ranellucci]

Hi all,

I'd like to extend the namespace policy for S3 target storage.
Bucket names are currently formatted as "<AccessKeyID>-backups" and
"<AccessKeyID>-chunks": this means that no more than a single dataset
can be stored on a S3 account. If we let the user specify a custom
prefix to use instead of <AccessKeyID> then we could have more logical
targets on a single S3 account, and thus multiple "-backups" and
"-chunks" folders.

The change involves just a "prefix" config option for S3 targets and it
would be backwards compatible.

Thoughts?

  - alessandro.