Fix bug with credential set on all gateways

General MR

Summary

Istio TLS Mode supports 5 different options for your gateway. Only MUTUAL and SIMPLE actually support certs/credentials though. This can be confirmed by applying a gateway like this:

apiVersion: networking.istio.io/v1beta1
kind: Gateway
metadata:
  name: passthrough
  namespace: istio-system
spec:
  selector:
    app: passthrough-ingressgateway
  servers:
  - hosts:
    - test.bigbang.dev
    port:
      name: https
      number: 8443
      protocol: HTTPS
    tls:
      credentialName: passthrough-cert
      mode: SIMPLE

And then swapping the mode to the other 4 options. PASSTHROUGH type modes will throw a warning that the credential will be ignored. ISTIO_MUTUAL will actually error out.