Restart kubelet if it does not report an internal/external ip address

On kubelet start-up some functions to set the node status are generated. One of those functions propagates the node addresses into the `Node` object the kubelet is responsible for (`.status.addresses`).
The kube-apiserver uses these addresses to talk to the actual node.

To identify the IP address of the node the kubelet communicates with the cloud provider. kubernetes/kubernetes#62543 introduced a timeout of 10s when trying to connect to the cloud. In case the IP cannot
be determined within 10s, the `Node` object does not report an `InternalIP` address.

Consequently, the kube-apiserver will never be able to talk to that node; particularly VPN won't work in case the vpn-shoot pod is scheduled on it.

Once the connection failed, it is never retried, and only a kubelet process restart can trigger it again. Hence, our kubelet monitoring script will now do the same when it cannot find an `InternalIP` or an `ExternalIP`
address on the `Node` object.

closes gardener#283

charts/shoot-cloud-config/charts/original/templates/scripts/_health-monitor.sh

@@ -48,8 +48,20 @@
           continue
         fi
 
+        node_status="$(kubectl get nodes -l kubernetes.io/hostname=$(hostname) -o json | jq -r '.items[0].status')"
+
+        # Check whether the kubelet does report an InternalIP node address
+        if node_ip_addresses="$(echo $node_status | jq -r '.addresses[] | select(.type=="InternalIP" or .type=="ExternalIP") | .address')"; then
+          if [[ -z "$node_ip_addresses" ]]; then
+            echo "Kubelet has not reported an InternalIP nor an ExternalIP node address yet. Restarting kubelet!";
+            restart_kubelet
+            sleep 20
+            continue
+          fi
+        fi
+
         # Check whether kubelet ready status toggles between true and false and reboot VM if happened too often.
-        if status="$(kubectl get nodes -l kubernetes.io/hostname=$(hostname) -o json | jq -r '.items[0].status.conditions[] | select(.type=="Ready") | .status')"; then
+        if status="$(echo $node_status | jq -r '.conditions[] | select(.type=="Ready") | .status')"; then
           if [[ "$status" != "True" ]]; then
             if [[ $time_kubelet_not_ready_first_occurrence == 0 ]]; then
               time_kubelet_not_ready_first_occurrence=$(date +%s)