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.

final Invoker<?> invoker;
            if (originInvoker instanceof InvokerDelegate) {
                invoker = ((InvokerDelegate<?>) originInvoker).getInvoker();
            } else {
                invoker = originInvoker;
            }
            
            URL originUrl = RegistryProtocol.this.getProviderUrl(invoker);
            String key = getCacheKey(originInvoker);
            ExporterChangeableWrapper<?> exporter = bounds.get(key);
            if (exporter == null) {
                logger.warn(new IllegalStateException("error state, exporter should not be null"));
                return;
            }
            
            URL currentUrl = exporter.getInvoker().getUrl();
            
            URL newUrl = getConfigedInvokerUrl(configurators, originUrl);
            newUrl = getConfigedInvokerUrl(serviceConfigurationListeners.get(originUrl.getServiceKey())
                    .getConfigurators(), newUrl);
            newUrl = getConfigedInvokerUrl(providerConfigurationListener.getConfigurators(), newUrl);
            if (!currentUrl.equals(newUrl)) {
                RegistryProtocol.this.reExport(originInvoker, newUrl);
                logger.info("exported provider url changed, origin url: " + originUrl +
                        ", old export url: " + currentUrl + ", new export url: " + newUrl);
            }
        }
        private List<URL> getMatchedUrls(List<URL> configuratorUrls, URL currentSubscribe) {
            List<URL> result = new ArrayList<URL>();
            for (URL url : configuratorUrls) {