avoid system.exit during shutdown test

Author: sureshanaparti

plugins/maintenance/src/test/java/org/apache/cloudstack/maintenance/ManagementServerMaintenanceManagerImplTest.java

@@ -114,6 +114,7 @@ public void cancelShutdown() {
     @Test
     public void triggerShutdown() {
         Mockito.doNothing().when(jobManagerMock).disableAsyncJobs();
+        Mockito.when(spy.isShutdownTriggered()).thenReturn(false);
         spy.triggerShutdown();
         Mockito.verify(jobManagerMock).disableAsyncJobs();