Kustomize installer: add Istio overlay

kustomize/istio/kustomization.yaml

@@ -0,0 +1,5 @@
+namespace: istio-system
+bases:
+  - ../base/flagger/
+patchesStrategicMerge:
+  - patch.yaml

kustomize/istio/patch.yaml

@@ -0,0 +1,15 @@
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: flagger
+spec:
+  template:
+    spec:
+      containers:
+        - name: flagger
+          args:
+            - -mesh-provider=istio
+            - -metrics-server=http://prometheus:9090
+            - -slack-user=flagger
+            - -slack-channel=
+            - -slack-url=