Break out the configuration for the prometheus fail2ban exporter, which
is a simple python script that exports stats from fail2ban as a
prometheus-scrapable textfile, from Mugwump into a reusable nixos module
in //ops/nixos/modules.
Change-Id: I5451c9c5de6c7bc4431150ae596a9c758bf1b693
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3136
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>
