Sentinel zk datasource add default server-addr

spring-cloud-alibaba-sentinel-datasource/src/main/java/com/alibaba/cloud/sentinel/datasource/config/ZookeeperDataSourceProperties.java

@@ -32,7 +32,7 @@ public class ZookeeperDataSourceProperties extends AbstractDataSourceProperties
 		super(ZookeeperDataSourceFactoryBean.class.getName());
 	}
 
-	private String serverAddr;
+	private String serverAddr = "localhost:2181";
 
 	private String path;