Add checks for the load-balanced mirrors

We have checks for each of the mirrors for each provider, but we had no checks that ensured the load balancer (vShield Edge) before the mirrors was functioning.

These checks are the result of a load balancer failing to pass traffic to either of its mirrors.

This load balancer failure has been temporarily circumvented by modifying DNS so that `www-origin.mirror.provider1.production.govuk.service.gov.uk` resolves to `mirror0.mirror.provider1.production.govuk.service.gov.uk`.