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.yml.j2

@@ -3,7 +3,7 @@ kind: ConfigMap
 apiVersion: v1
 metadata:
   name: kube-flannel-cfg
-  namespace: "{{system_namespace}}"
+  namespace: "kube-system"
   labels:
     tier: node
     app: flannel
@@ -41,7 +41,7 @@ apiVersion: extensions/v1beta1
 kind: DaemonSet
 metadata:
   name: kube-flannel
-  namespace: "{{system_namespace}}"
+  namespace: "kube-system"
   labels:
     tier: node
     k8s-app: flannel