Fix SSH polluting known_hosts

In case NoHostAuthenticationForLocalhost is not specified in user's SSH
configuration, starting the SSH daemon will write to the known_hosts
file, potentially leading to conflicts later when the same port is
reused.
Test the SSH connection with the option on, but without touching the
user's configuration.
Use Dropbear instead of OpenSSH
Merge pull request #36 from koterpillar/dropbear

Use Dropbear instead of OpenSSH
Merge pull request #37 from koterpillar/fix-ssh-polluting-known-hosts

Fix SSH polluting known_hosts