Salt preflight script could be attacker controlled

The Salt-SSH pre-flight option copies the script to the target at a predictable path, which allows an attacker to force Salt-SSH to run their script. If an attacker has access to the target VM and knows the path to the pre-flight script before it runs they can ensure Salt-SSH runs their script with the privileges of the user running Salt-SSH. Do not make the copy path on the target predictable and ensure we check return codes of the scp command if the copy fails.