Change order of sshconfig.Auth

Change order of ssh key's files inclusion for sshconfig.Auth.

Why ? Because the first instance of a AuthMethod is used during authentification (~/.ssh/id_rsa by default) and the other one will be ignored. (https://godoc.org/code.google.com/p/go.crypto/ssh#ClientConfig)

So, in my opinion, defined ssh key's files have higher priority than default one.