Fix ssh config by letting fabric do it

Fix for issue #146.

Remove littlechef's credentials function, as fabric natively handles these functions with recent releases. Littlechef's attempts were conflicting with fabric's native functionality.

Tested with Fabric 1.4.2 (minimum version supported by littlechef) and 1.5.3 (most recent version).

Tested authentication scenarios include username, password and keypair provided in config.cfg, and Host, HostName, User, and IdentityFile provided in an ssh configuration specified by ssh-config.