Configure a reverse SSH tunnel from the public compbox

[PeterJCLaw]

This provides a way to remotely administer the kiosk devices without needing them to be on the same network.

Based on guidance at https://www.everythingcli.org/ssh-tunnelling-for-fun-and-profit-autossh/

TODO:

• assign ports to each Pi (ideally based on something in the pi_macs file)
• apply to all pis
• add generated ssh keys to https://github.com/PeterJCLaw/srcomp-puppet/blob/master/modules/compbox/files/autossh-authorized_keys

[PeterJCLaw]

cc @sedders123