Use an s390x default-http-backend

cluster/juju/layers/kubernetes-worker/reactive/kubernetes_worker.py

@@ -605,6 +605,12 @@ def launch_default_ingress_controller():
     context['arch'] = arch()
     addon_path = '/root/cdk/addons/{}'
 
+    context['defaultbackend_image'] = \
+        "gcr.io/google_containers/defaultbackend:1.4"
+    if arch() == 's390x':
+        context['defaultbackend_image'] = \
+            "gcr.io/google_containers/defaultbackend-s390x:1.4"
+
     # Render the default http backend (404) replicationcontroller manifest
     manifest = addon_path.format('default-http-backend.yaml')
     render('default-http-backend.yaml', manifest, context)

cluster/juju/layers/kubernetes-worker/templates/default-http-backend.yaml

@@ -17,7 +17,7 @@ spec:
         # Any image is permissable as long as:
         # 1. It serves a 404 page at /
         # 2. It serves 200 on a /healthz endpoint
-        image: gcr.io/google_containers/defaultbackend:1.0
+        image: {{ defaultbackend_image }}
         livenessProbe:
           httpGet:
             path: /healthz