Add support for changing HostKey option

This is to make it possible to change the HostKey parameter in
sshd_config.

As the HostKey is hardcoded to the template now it is currently not
possible to connect using for example ssh-dss. This commit changes that
behaviour.

Please let me know if you have any suggestions or comments about the changes. I'm not sure if there's a more elegant way of doing the path validation.