cluster.tf: Add additional explicit dependencies
To avoid the following error during setup of a fresh cluster, make
docker-hub-mirror (and reggie) depend on the ingress module, and make
the ingress module depend on the istio module.
Error: Internal error occurred: failed calling webhook "validate.nginx.ingress.kubernetes.io": failed to call webhook: Post "https://ingress-nginx-controller-admission.ingress.svc:443/networking/v1/ingresses?timeout=10s": http: server gave HTTP response to HTTPS client
with module.docker-hub-mirror.helm_release.docker-hub-mirror,
on docker-hub-mirror/main.tf line 1, in resource "helm_release" "docker-hub-mirror":
1: resource "helm_release" "docker-hub-mirror" {