we should insert alert DB once , and trigger this type of alert 3 times (#1622)

dolphinscheduler-server/src/main/java/org/apache/dolphinscheduler/server/master/MasterServer.java

@@ -152,11 +152,9 @@ public class MasterServer implements IStoppable {
             @Override
             public void run() {
                 if (zkMasterClient.getActiveMasterNum() <= 1) {
-                    for (int i = 0; i < Constants.DOLPHINSCHEDULER_WARN_TIMES_FAILOVER; i++) {
                     zkMasterClient.getAlertDao().sendServerStopedAlert(
                         1, OSUtils.getHost(), "Master-Server");
                 }
-                }
                 stop("shutdownhook");
             }
         }));

dolphinscheduler-server/src/main/java/org/apache/dolphinscheduler/server/worker/WorkerServer.java

@@ -177,10 +177,8 @@ public class WorkerServer implements IStoppable {
             public void run() {
                 // worker server exit alert
                 if (zkWorkerClient.getActiveMasterNum() <= 1) {
-                    for (int i = 0; i < Constants.DOLPHINSCHEDULER_WARN_TIMES_FAILOVER; i++) {
                     alertDao.sendServerStopedAlert(1, OSUtils.getHost(), "Worker-Server");
                 }
-                }
                 stop("shutdownhook");
             }
         }));

dolphinscheduler-server/src/main/java/org/apache/dolphinscheduler/server/zk/ZKMasterClient.java

@@ -248,10 +248,8 @@ public class ZKMasterClient extends AbstractZKClient {
 	private void alertServerDown(String serverHost, ZKNodeType zkNodeType) {
 
 		String serverType = zkNodeType.toString();
-		for (int i = 0; i < Constants.DOLPHINSCHEDULER_WARN_TIMES_FAILOVER; i++) {
 		alertDao.sendServerStopedAlert(1, serverHost, serverType);
 	}
-	}
 
 	/**
 	 * monitor master