[#67217222] Add a Varnish cache health check
[#67217222] Add a Varnish cache health check

Add a Varnish health check that returns a HTTP 200 'OK' message when
queried on the /__varnish_check__ endpoint.

This is in preparation for setting the vShield Edge Load Balancers to
use this endpoint as a health check, so that the load balancers can
failover if varnish is not responding.

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