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.
Merge pull request #85 from diddi-/sshd_hostkey

Add support for changing HostKey option in sshd_config