Co-Authored-By: Jean-Paul Calderone <jean-paul@privatestorage.io>

This also update ZKAPAuthorizer to a python3 using version.

Thanks @jcalderone for pointing me to it

Else the script will fail if the links are already there and not
rotate the preshared.key.

Also now write out a new preshared.key before creating the links
just in case that flag gets lost again.