chore: switch to layered helmfiles for istio

Basically the same migration as in c7eb072

helmfile was giving us warnings because the environments where not
seperate yaml documents. Also the recommendation how to structure you
helmfile changed. There is a new layering featurte that enabled you to
share common parts (like environments ;))

infrastructure/istio/helmfile.d/00-namespaces.yaml

@@ -1,6 +1,5 @@
-environments:
-  default:
-  cni:
+bases:
+  - environments.yaml
 
 repositories:
   - name: incubator

infrastructure/istio/helmfile.d/05-istio-base.yaml

@@ -1,6 +1,5 @@
-environments:
-  default:
-  cni:
+bases:
+  - environments.yaml
 
 repositories:
   - name: istio

infrastructure/istio/helmfile.d/06-istio-cni.yaml

@@ -1,6 +1,5 @@
-environments:
-  default:
-  cni:
+bases:
+  - environments.yaml
 
 repositories:
   - name: istio

infrastructure/istio/helmfile.d/10-istiod.yaml

@@ -1,6 +1,5 @@
-environments:
-  default:
-  cni:
+bases:
+  - environments.yaml
 
 repositories:
   - name: istio

infrastructure/istio/helmfile.d/11-istio-ingress.yaml

@@ -1,6 +1,5 @@
-environments:
-  default:
-  cni:
+bases:
+  - environments.yaml
 
 repositories:
   - name: istio

infrastructure/istio/helmfile.d/20-addons.yaml

@@ -1,6 +1,5 @@
-environments:
-  default:
-  cni:
+bases:
+  - environments.yaml
 
 repositories:
   - name: prometheus-community

infrastructure/istio/helmfile.d/environments.yaml

@@ -0,0 +1,3 @@
+environments:
+  default:
+  cni: