Treat sigterm as a clean shutdown signal (#4277)

cmd/telegraf/telegraf.go

@@ -147,11 +147,11 @@ func reloadLoop(
 
 		shutdown := make(chan struct{})
 		signals := make(chan os.Signal)
-		signal.Notify(signals, os.Interrupt, syscall.SIGHUP)
+		signal.Notify(signals, os.Interrupt, syscall.SIGHUP, syscall.SIGTERM)
 		go func() {
 			select {
 			case sig := <-signals:
-				if sig == os.Interrupt {
+				if sig == os.Interrupt || sig == syscall.SIGTERM {
 					close(shutdown)
 				}
 				if sig == syscall.SIGHUP {