fix: istio gateway and ttl destination customization

[moabu]

We'v2 customization to the helm charts need to be addressees:

• /charts/oxshibboleth/templates/oxshibboleth-virtual-services.yaml

This chart does not have the logic to use an external Istio gateway resource and by default is trying to bind to the default gluu-global-gtw. Please add the below snippet to the chart file:

{{- if .Values.global.istio.gateways }}
{{- with .Values.global.istio.gateways }}
{{- toYaml . | nindent 4 }}
{{- end }}
{{- else }}
  - {{ .Release.Name }}-global-gtw
{{- end }}

• /charts/oxshibboleth/templates/oxshibboleth-destination-rules.yaml

  TTL value should be read from the values.yaml to make custom values easier to manage and remove the risk of reverting the value to 60s.