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.

import static org.apache.dubbo.common.Constants.QOS_PORT;
import static org.apache.dubbo.common.Constants.REFER_KEY;
import static org.apache.dubbo.common.Constants.REGISTER_IP_KEY;
import static org.apache.dubbo.common.Constants.REGISTER_KEY;
import static org.apache.dubbo.common.Constants.REGISTRY_KEY;
import static org.apache.dubbo.common.Constants.REGISTRY_PROTOCOL;
import static org.apache.dubbo.common.Constants.ROUTERS_CATEGORY;
import static org.apache.dubbo.common.Constants.SIMPLIFIED_KEY;
import static org.apache.dubbo.common.Constants.VALIDATION_KEY;
import static org.apache.dubbo.common.utils.UrlUtils.classifyUrls;
/**
 * RegistryProtocol
 */
public class RegistryProtocol implements Protocol {
    private final static Logger logger = LoggerFactory.getLogger(RegistryProtocol.class);
    private static RegistryProtocol INSTANCE;
    private final Map<URL, NotifyListener> overrideListeners = new ConcurrentHashMap<>();
    private final Map<String, ServiceConfigurationListener> serviceConfigurationListeners = new ConcurrentHashMap<>();
    private final ProviderConfigurationListener providerConfigurationListener = new ProviderConfigurationListener();
    
    
    private final ConcurrentMap<String, ExporterChangeableWrapper<?>> bounds = new ConcurrentHashMap<>();
    private Cluster cluster;
    private Protocol protocol;
    private RegistryFactory registryFactory;
    private ProxyFactory proxyFactory;
    public RegistryProtocol() {
        INSTANCE = this;
    }
    public static RegistryProtocol getRegistryProtocol() {