Add an Icinga alert for high queue levels on saatchi
The Rabbitmq instance on saatchi.internal.softwareheritage.org is graphed in Munin: http://munin.internal.softwareheritage.org/internal.softwareheritage.org/saatchi.internal.softwareheritage.org/rabbitmq_messages.html
Howewer, no alert is sent when an unreasonable number of messages is in the queue. Add an Icinga2 alert when 200K messages are being queued.
Migrated from T987 (view on Phabricator)