* Redefine SCP interface
* Implement Machine.scp_to and Machine.scp_from
* Implement Unit.scp_to and Unit.scp_from
* Fix Unit.machine to better handle missing machines
* Use os.path.expanduser for scp key path
* Use local-cloud address if public address is not found
* Rename Machine.public_address to Machine.dns_name to match CLI
* fix scp tests not waiting long enough