Droplet backend support

[Joacchim]

Currently, the tool is too rigid in the way it supports the multiple backends from droplet. For instance, the POSIX droplet backend cannot be supported because it does not provide any bucket functionnality, feature over which the internals of cloudmig rely heavily.

The solution may be linked to the possibility to migrate from a directory to a bucket, from a bucket to a directory, and between directories within some buckets.