- [x] I have searched the [issues](https://github.com/apache/dubbo/issues) of this repository and believe that this is not a duplicate.
- [x] I have checked the [FAQ](https://github.com/apache/dubbo/blob/master/FAQ.md) of this repository and believe that this is not a duplicate.

### Environment

* Dubbo version: 2.7.2
* Operating System version: macOS 10.12.6
* Java version: 1.8

### Steps to reproduce this issue



Pls. provide [GitHub address] to reproduce this issue.

### Expected Result

What do you expected from the above steps？

```java

    ZookeeperDynamicConfiguration(URL url, ZookeeperTransporter zookeeperTransporter) {
        this.url = url;
        rootPath = "/" + url.getParameter(CONFIG_NAMESPACE_KEY, DEFAULT_GROUP) + "/config";

        initializedLatch = new CountDownLatch(1);
        this.cacheListener = new CacheListener(rootPath, initializedLatch);
        this.executor = Executors.newFixedThreadPool(1, new NamedThreadFactory(this.getClass().getSimpleName(), true));

        zkClient = zookeeperTransporter.connect(url);
        zkClient.addDataListener(rootPath, cacheListener, executor);
        try {
            // Wait for connection
            this.initializedLatch.await();
        } catch (InterruptedException e) {
            logger.warn("Failed to build local cache for config center (zookeeper)." + url);
        }
    }
```

`        zkClient = zookeeperTransporter.connect(url);`会通过URL中的client参数来正确选择自适应扩展来初始化zk客户端。但在初始化配置中心的时候，URL是由ConfigCenterConfig得到的，此时URL中无client的信息，所以这个地方创建zk还是用的默认的zkClient。希望和注册中心时候一样获取自己扩展的zkClient


### Actual Result

What actually happens?

If there is an exception, please attach the exception trace:

```
Just put your stack trace here!
```

this.local = local;
    }
    public String getStub() {
        return stub;
    }
    public void setStub(Boolean stub) {
        if (stub == null) {
            setStub((String) null);
        } else {
            setStub(stub.toString());
        }
    }
    public void setStub(String stub) {
        checkName("stub", stub);
        this.stub = stub;
    }
    public String getCluster() {
        return cluster;
    }
    public void setCluster(String cluster) {
        checkExtension(Cluster.class, CLUSTER_KEY, cluster);
        this.cluster = cluster;
    }
    public String getProxy() {
        return proxy;
    }
    public void setProxy(String proxy) {
        checkExtension(ProxyFactory.class, PROXY_KEY, proxy);
        this.proxy = proxy;
    }