Refactor ssh properties

This PR pulls out all String references into a new `SshProperties` class.
No, it isn't a lazy question!  I am not aware of any and that's why I added it. :) My opinion is that it provides more context than the string `"jclouds.ssh"`. That said, _should_ we have a single place where they are all defined?  Is there perceived value here or should we just nuke it?