loadtest-controller: fix context null in shutdown manager

Author: ivchicano

loadtest-controller/src/main/java/io/openvidu/loadtest/utils/ShutdownManager.java

@@ -8,6 +8,10 @@
 public class ShutdownManager {
     private ApplicationContext context;
 
+    public ShutdownManager(ApplicationContext context) {
+        this.context = context;
+    }
+
     public void shutdownWithCode(int code) {
         SpringApplication.exit(context, () -> code);
     }