Stop templating kube-system namespace and creating it (#2545)

Kubernetes makes this namespace automatically, so there is
no need for kubespray to manage it.

roles/network_plugin/flannel/templates/cni-flannel-rbac.yml.j2

@@ -3,7 +3,7 @@ apiVersion: v1
 kind: ServiceAccount
 metadata:
   name: flannel
-  namespace: "{{system_namespace}}"
+  namespace: "kube-system"
 ---
 kind: ClusterRole
 apiVersion: rbac.authorization.k8s.io/v1beta1
@@ -41,4 +41,4 @@ roleRef:
 subjects:
 - kind: ServiceAccount
   name: flannel
-  namespace: "{{system_namespace}}"
+  namespace: "kube-system"