Create a DataProvider class using only SFTP for transport (Redmine 2173)

[prioux]

A new Data Provider class based only on the SFTP protocol would add another layer of security: if the data provider side is configured with a UNIX username that does NOT have a shell account, but only support the SFTP subsystem, then a comprised remote resource would never be able to let in the attacker through standard SSH.

This can be combined with a 'smart' DP, too, like the normal SSH.