engine: enrich ssh schema

[sergkudinov]

Define SSH properties in the scema of nikita.ssh.open().

Don't forget to update the docs page if something changed.

The full list from old Nikita

The [ssh2] configuration properties:

• host (string)
  Hostname or IP address of the remote server, default is localhost.

• port (integer)
  Port number of the server. Default: 22

• hostHash (string)
  'md5' or 'sha1'. The host's key is hashed using this method and passed to the hostVerifier function. Default: (none)

• hostVerifier (function)
  Function that is passed a string hex hash of the host's key for verification purposes. Return true to continue with the connection, false to reject and disconnect. Default: (none)

• username (string)
  Username for authentication. Default: (none)

• password (string)
  Password for password-based user authentication. Default: (none)

• agent (string)
  Path to ssh-agent's UNIX socket for ssh-agent-based user authentication. Windows users: set to 'pageant' for authenticating with Pageant or (actual) path to a cygwin "UNIX socket." Default: (none)

• privateKey (mixed)
  Buffer or string that contains a private key for key-based user authentication (OpenSSH format). Default: (none)

• passphrase (string)
  For an encrypted private key, this is the passphrase used to decrypt it. Default: (none)

• tryKeyboard (boolean)
  Try keyboard-interactive user authentication if primary user authentication method fails. Default: false

• pingInterval (integer)
  How often (in milliseconds) to send SSH-level keepalive packets to the server. Default: 60000

• readyTimeout (integer)
  How long (in milliseconds) to wait for the SSH handshake to complete. Default: 10000

• sock (ReadableStream)
  A ReadableStream to use for communicating with the server instead of creating and using a new TCP connection (useful for connection hopping).

• agentForward (boolean)
  Set to true to use OpenSSH agent forwarding ('auth-agent@openssh.com'). Default: false

The [ssh2-connect] config:

• username (string)
  The username used to initiate the connection, default to the current
  environment user.
• privateKeyPath (string)
  Path to the file containing the private key.
• retry (integer)
  Attempt to reconnect multiple times, default to "1".
• wait (integer)
  Time to wait in milliseconds between each retry, default to "500".