fix mailman queue monitoring

follow-up to Change-Id: Ife9e2e4544b3495

currently doesn't work, status UNKNOWN

- this needs to be an NRPE check, not a regular one
- usually with the new method to setup NRPE checks,
  we just define the checkcommands and are done,
  because they use stuff from the nagios-plugins package
- but in this case it's a custom plugin, so that needs
  to be installed somehow as well (on sodium! not on neon)

https://icinga.wikimedia.org/cgi-bin/icinga/extinfo.cgi?type=2&host=sodium&service=mailman_queue_size

Change-Id: If03d81ce1b0b7675f8a428cb328d3aee9b43d4c8
Merge "fix mailman queue monitoring" into production