Fix #798 - RBAC for leader election

Using gcr.io/google_containers/nginx-ingress-controller:0.9.0-beta.7
the nginx-controller needs to handle leader-election via configmaps.

To perform the leader-election the nginx-controller needs to have the
appropiate RBAC permissions.

Previously to this fix, the following errors occured:

-  cannot get configmaps in the namespace "NAMESPACE_PLACEHOLDER". (get configmaps ingress-controller-leader-nginx)
- initially creating leader election record: User "system:serviceaccount:NAMESPACE_PLACEHOLDER" cannot create configmaps in the namespace "NAMESPACE_PLACEHOLDER". (post configmaps)

fix ingress rbac roles

There was 2 things that the current IC (0.9 beta7) needs.

The ClusterRole was missing `get nodes`:

```
RBAC DENY: user "system:serviceaccount:kube-system:nginx-ingress-controller" groups [system:serviceaccounts system:serviceaccounts:kube-system system:authenticated] cannot "get" resource "nodes" named "xxx" cluster-wide
```

The Role was missing `update configmaps`:

```RBAC DENY: user "system:serviceaccount:kube-system:nginx-ingress-controller" groups [system:serviceaccounts system:serviceaccounts:kube-system system:authenticated] cannot "update" resource "configmaps" named "ingress-controller-leader-nginx" in namespace "kube-system"```

removed update configmap because of #798

rebased on master, moved get nodes to own rule

added get nodes to cluster permissions

examples/rbac/nginx/nginx-ingress-controller-rbac.yml

@@ -25,6 +25,12 @@ rules:
     verbs:
       - list
       - watch
+  - apiGroups:
+      - ""
+    resources:
+      - nodes
+    verbs:
+      - get
   - apiGroups:
       - ""
     resources:
@@ -69,6 +75,20 @@ rules:
       - secrets
     verbs:
       - get
+  - apiGroups:
+      - ""
+    resources:
+      - configmaps
+    resourceNames:
+      # Defaults to "<election-id>-<ingress-class>"
+      # Here: "<ingress-controller-leader>-<nginx>"
+      # This has to be adapted if you change either parameter
+      # when launching the nginx-ingress-controller.
+      - "ingress-controller-leader-nginx"
+    verbs:
+      - create
+      - get
+      - update
   - apiGroups:
       - ""
     resources: