tiny refactor: just move around some code

The router name derivation uses the result of createOverlay, so move
it closer to that.

prog/weaver/main.go

@@ -133,24 +133,13 @@ func main() {
 	overlay, bridge := createOverlay(datapathName, ifaceName, config.Port, bufSzMB)
 	networkConfig.Bridge = bridge
 
-	if password == "" {
-		password = os.Getenv("WEAVE_PASSWORD")
-	}
-	if password == "" {
-		Log.Println("Communication between peers is unencrypted.")
-	} else {
-		config.Password = []byte(password)
-		Log.Println("Communication between peers via untrusted networks is encrypted.")
-	}
-
 	if routerName == "" {
-		iface := networkConfig.Bridge.Interface()
+		iface := bridge.Interface()
 		if iface == nil {
 			Log.Fatal("Either an interface must be specified with --datapath or --iface, or a name with --name")
 		}
 		routerName = iface.HardwareAddr.String()
 	}
-
 	name, err := mesh.PeerNameFromUserInput(routerName)
 	checkFatal(err)
 
@@ -159,6 +148,16 @@ func main() {
 		checkFatal(err)
 	}
 
+	if password == "" {
+		password = os.Getenv("WEAVE_PASSWORD")
+	}
+	if password == "" {
+		Log.Println("Communication between peers is unencrypted.")
+	} else {
+		config.Password = []byte(password)
+		Log.Println("Communication between peers via untrusted networks is encrypted.")
+	}
+
 	if prof != "" {
 		p := *profile.CPUProfile
 		p.ProfilePath = prof