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/weave/templates/weave-net.yml.j2

@@ -8,14 +8,14 @@ items:
       name: weave-net
       labels:
         name: weave-net
-      namespace: {{ system_namespace }}
+      namespace: kube-system
   - apiVersion: rbac.authorization.k8s.io/v1beta1
     kind: ClusterRole
     metadata:
       name: weave-net
       labels:
         name: weave-net
-      namespace: {{ system_namespace }}
+      namespace: kube-system
     rules:
       - apiGroups:
           - ''
@@ -41,7 +41,7 @@ items:
       name: weave-net
       labels:
         name: weave-net
-      namespace: {{ system_namespace }}
+      namespace: kube-system
     roleRef:
       kind: ClusterRole
       name: weave-net
@@ -49,14 +49,14 @@ items:
     subjects:
       - kind: ServiceAccount
         name: weave-net
-        namespace: {{ system_namespace }}
+        namespace: kube-system
   - apiVersion: rbac.authorization.k8s.io/v1beta1
     kind: Role
     metadata:
       name: weave-net
       labels:
         name: weave-net
-      namespace: {{ system_namespace }}
+      namespace: kube-system
     rules:
       - apiGroups:
           - ''
@@ -79,7 +79,7 @@ items:
       name: weave-net
       labels:
         name: weave-net
-      namespace: {{ system_namespace }}
+      namespace: kube-system
     roleRef:
       kind: Role
       name: weave-net
@@ -87,7 +87,7 @@ items:
     subjects:
       - kind: ServiceAccount
         name: weave-net
-        namespace: {{ system_namespace }}
+        namespace: kube-system
   - apiVersion: extensions/v1beta1
     kind: DaemonSet
     metadata:
@@ -95,7 +95,7 @@ items:
       labels:
         name: weave-net
         version: v{{ weave_version }}
-      namespace: {{ system_namespace }}
+      namespace: kube-system
     spec:
       minReadySeconds: 5
       template: