ingress-nginx controller - configuration injection via unsanitized mirror annotations

A security issue was discovered in ingress-nginx https://github.com/kubernetes/ingress-nginx where the `mirror-target` and `mirror-host` Ingress annotations can be used to inject arbitrary configuration into nginx. This can lead to arbitrary code execution in the context of the ingress-nginx controller, and disclosure of Secrets accessible to the controller. (Note that in the default installation, the controller can access all Secrets cluster-wide.)