Specify which `known_hosts` file to use (or disable host key checking)

[jefmathiot]

Three use cases:

• the user may want to reuse a specific know_hosts file, for example created with OpenSSH
• by default, use the current user's know_hosts
• disable host key checking (not a good practice, but may be useful for testing)

[jefmathiot]

Proposed net-ssh/net-ssh#250 to allow ES.io to read hostname hashes in known_hosts files.