[storage] Swift and S3 driver does not have the same upload behaviour

[EtienneM]

When using the Upload method, on Swift we remove the prefix / (https://github.com/Scalingo/go-utils/blob/master/storage/swift.go#L120):

func (s *Swift) fullPath(path string) string {
	return strings.TrimLeft(s.cfg.Prefix+"/"+path, "/")
}

But we don't do the same with S3. Hence for the same code, when I switch from one driver to another I don't have the same upload behaviour. It's annoying as with s3cmd, here is what I saw:

╰─$ s3cmd ls s3://auditlogs-development                                                                                                     
                       DIR   s3://auditlogs-development//

Then I need to write the following to see my upload:

╰─$ s3cmd ls s3://auditlogs-development//           
                       DIR   s3://auditlogs-development//2019-08-26/

It confused me...

@Soulou Don't you think we should have the same behaviour for both? Do you prefer to remove this behaviour from Swift or add it to S3?