Get mongo log into kibana

Mongo outputs a log file. It's not JSON, but it would be very helpful to get
it into Kibana to diagnose problems on the system.

Each line should end up in the @message field and the @tags should be set to
['mongo'].

See https://github.com/alphagov/puppet-lumberjack/blob/963590c087433819e396db96eafab7f8eb2f101f/manifests/logshipper.pp