RFE: Check permissions before uploading big files

[reidrac]

The communication between the SFTP daemon and Swift is over HTTP, and it means that when uploading a big file without write permissions we need to upload the whole file before getting an error.

It would be a good idea testing the permissions first (ie. create a zero bytes file), so the SFTP service returns the error ASAP.

[reidrac]

After a quick look I think this should be implemented on the FS level, and that means on ftp-cloudfs (well, we need to factorize the fs code out of that project).

CloudFilesFS.open should try to create a zero bytes object when mode is in ["w", "rw"] before returning the CloudFilesFD object.