Harden login to DANSE server

[butlerpd]

UTK is working hard to enhance its overall IT security posture. As such it would like to restrict ssh access to all servers in the domain to computers in the same domain. The DANSE server needs an exception due to the nature of the work however, we can do some things to enhance security. Namely:

• Add fail2ban to prevent brute force attacks (see for example: https://www.digitalocean.com/community/tutorials/how-to-protect-ssh-with-fail2ban-on-ubuntu-14-04)
• Require 2FA and/or key access