(security) Optionally only allow MAAS api access

- MAAS does not allow you to turn off the gui which
  may be desired in some scenarios. Use Ingress rules
  to do so optionally.

Change-Id: I22f637ebd2dbbd7c552fd4644bcf27cc9b9661d8

charts/maas/templates/ingress-region.yaml

@@ -27,7 +27,11 @@ spec:
     - host: {{ tuple "maas_region" "public" . | include "helm-toolkit.endpoints.hostname_fqdn_endpoint_lookup" }}
       http:
         paths:
+{{- if .Values.conf.maas.ingress_disable_gui }}
+          - path: /MAAS/api
+{{- else }}
           - path: /
+{{- end }}
             backend:
               serviceName: {{ tuple "maas_region" "internal" . | include "helm-toolkit.endpoints.hostname_short_endpoint_lookup" }}
               servicePort: region-api

charts/maas/values.yaml

@@ -154,6 +154,7 @@ conf:
     override:
     append:
     http_boot: true
+    ingress_disable_gui: false
     ntp:
       # These options allow you to mock out the ntpd binary within the container
       # by overwriting it with a script that simply sleeps - this is useful in