When directory_path is set the wrong protocol is used

[simahawk]

If you set directory_path while using SFTP for instance, the fs object won't be the one of the protocol you've chosen.
In storage_backend you can set a default root path for all type of storages.

It will automatically - and w/o notice - use RootedDirFileSystem. This leads to broken implementations because it will raise NotImplementedError when trying to use sftp api.

It would be nice to support this in the future. Meanwhile we should probably ignore that param and/or hide it or raise warning.

To reproduce:

1. add an sftp storage
2. set a directory_path
3. try to move a file
4. kaboom