SFTP as storage provider

[MaticSulc]

Since we have FTP as a storage provider, I believe it would make sense to have SFTP support as well. I am not sure if the SSL checkbox makes the storage provider act as SFTP in the background.

It would however be really useful to have SSH key authentication - it shouldn't be hard to set up, since we already have keys available inside Vito.

[saeedvaziry]

#989