Manage the ssh_known_hosts when we use exported sshkeys

This makes sure the file is readable by every user. I had a case on Ubuntu 14.04 where this wasn't the case.

In addition allow the user to configure the ssh_known_hosts filename.
This seems odd, since the sshkey should do this for us. Was the issue that you had that the ssh_known_hosts file did not exist or had bad permissions?