Known Hosts fingerprint verification

[Apreche]

The user is only required to provide the known_hosts file when using a jump host. They are permitted to provide one for direct connections. If it is not provided, then we automatically create one. That's the equivalent of what SSH does if the user says "yes" at the prompt. For security reasons we should probably exactly mimic the SSH client behavior of showing the user the fingerprint and prompting them to verify it with a yes or no.