Add logstash filter for logrus logging
Add logstash filter for logrus logging

Apps that use Go's logrus [1] (all of our Go apps) output a `msg`
field which contains the log message. Logstash/Kibana uses an `@message`
field.

This commit adds a rename filter to Logstash so that we have more consistent
naming of that field.

[1]: https://github.com/Sirupsen/logrus

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet