fix example configuration with hostkeys

[jrse]

The sample configuration for hostkey auth. does not work out of the box. it's better to add the full example and test command here.

[atmoz]

The mounting of host public key can be confusing for people, since it is not necessary for host keys to work. It's also not a good idea to mix host keys with client keys like in the example where the private key for the host is used by the client to login.

To test if the host keys work all that is needed is to login and verify that the host key fingerprint is correct.