Generate an SSH keypair with
keygen(comment, path, callback(err))
Generate the keys at the given path.
path will be the private, and
path + '.pub' will be the public key.
keygen(comment, options, callback(err))
options object with fields
path (used as above) and
determines the type of keys generated - default "rsa", with "ecdsa" being a good
option if your system supports it).
keygen(comment, callback(err, privkey, pubkey))
Generate the keys to a random path and return them as strings.