[#58082150] Nagios alert for Router healthcheck
[#58082150] Nagios alert for Router healthcheck

Configure an "app is running" Nagios check against the Router's healtcheck
endpoint on the API interface. Code duplicated here instead of `govuk::app`
per comment.

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