Subject:- Canary support for istio multi cluster

Body:-
If applied this commit will resolve the following reported issue https://github.com/weaveworks/flagger/issues/437
Have added support for consuming kubeconfig of istio host cluster where istio
resources will be created.

cmd/flagger/main.go

@@ -58,6 +58,7 @@ var (
 	leaderElectionNamespace  string
 	enableConfigTracking     bool
 	ver                      bool
+	kubeconfigHost           string
 )
 
 func init() {
@@ -83,6 +84,7 @@ func init() {
 	flag.StringVar(&leaderElectionNamespace, "leader-election-namespace", "kube-system", "Namespace used to create the leader election config map.")
 	flag.BoolVar(&enableConfigTracking, "enable-config-tracking", true, "Enable secrets and configmaps tracking.")
 	flag.BoolVar(&ver, "version", false, "Print version")
+	flag.StringVar(&kubeconfigHost, "kubeconfig-host", "", "Path to a kubeconfig for host cluster. Only required if cluster has a host cluster.")
 }
 
 func main() {
@@ -117,7 +119,16 @@ func main() {
 		logger.Fatalf("Error building kubernetes clientset: %v", err)
 	}
 
-	meshClient, err := clientset.NewForConfig(cfg)
+	// if host kube config is there than this should be spawned with host kubeconfig
+	cfgHost, err := cfg, nil
+	if kubeconfigHost != "" {
+		cfgHost, err = clientcmd.BuildConfigFromFlags(masterURL, kubeconfigHost)
+		if err != nil {
+			logger.Fatalf("Error building host kubeconfig: %v", err)
+		}
+	}
+
+	meshClient, err := clientset.NewForConfig(cfgHost)
 	if err != nil {
 		logger.Fatalf("Error building mesh clientset: %v", err)
 	}