When I set `path` in ServiceConfig or `contextpath` in ProtocolConfig, RestProtocol throws NPE on this line:
https://github.com/apache/incubator-dubbo/blob/master/dubbo-rpc/dubbo-rpc-rest/src/main/java/org/apache/dubbo/rpc/protocol/rest/RestProtocol.java#L93
```java
Class implClass = ApplicationModel.getProviderModel(url.getServiceKey()).getServiceInstance().getClass();
```

<dubbo:service path="test" interface="org.apache.dubbo.samples.rest.api.facade.UserRestService" ref="userRestService"
                   protocol="rest" validation="true"/>

This is because we use `group/interface:version` as the key when putting ProviderModel:
```java
Application.initProviderModel(group/interface:version, providerModel)
```
but use `group/path:version` to get, as showed above.

I think to unify to `path` would solve this problem.

return 9090;
    }
    public Map<URL, NotifyListener> getOverrideListeners() {
        return overrideListeners;
    }
    public void register(URL registryUrl, URL registeredProviderUrl) {
        Registry registry = registryFactory.getRegistry(registryUrl);
        registry.register(registeredProviderUrl);
    }
    public void unregister(URL registryUrl, URL registeredProviderUrl) {
        Registry registry = registryFactory.getRegistry(registryUrl);
        registry.unregister(registeredProviderUrl);
    }
    @Override
    public <T> Exporter<T> export(final Invoker<T> originInvoker) throws RpcException {
        URL registryUrl = getRegistryUrl(originInvoker);
        
        URL providerUrl = getProviderUrl(originInvoker);
        
        
        
        
        final URL overrideSubscribeUrl = getSubscribedOverrideUrl(providerUrl);
        final OverrideListener overrideSubscribeListener = new OverrideListener(overrideSubscribeUrl, originInvoker);
        overrideListeners.put(overrideSubscribeUrl, overrideSubscribeListener);
        providerUrl = overrideUrlWithConfig(providerUrl, overrideSubscribeListener);
        
        final ExporterChangeableWrapper<T> exporter = doLocalExport(originInvoker, providerUrl);
        
        final Registry registry = getRegistry(originInvoker);