fix canary backend health check
fix canary backend health check

The canary backend health check is not working. Fortunately, AWS forwards traffic to all targets if all targets are unhealthy but when new backend machine come up, some backend are actually not healthy and caused pingdom to fail.

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