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.

URL overrideUrl = url;
                
                if (url.getParameter(CATEGORY_KEY) == null && OVERRIDE_PROTOCOL.equals(url.getProtocol())) {
                    overrideUrl = url.addParameter(CATEGORY_KEY, CONFIGURATORS_CATEGORY);
                }
                
                if (UrlUtils.isMatch(currentSubscribe, overrideUrl)) {
                    result.add(url);
                }
            }
            return result;
        }
    }
    private class ServiceConfigurationListener extends AbstractConfiguratorListener {
        private URL providerUrl;
        private OverrideListener notifyListener;
        public ServiceConfigurationListener(URL providerUrl, OverrideListener notifyListener) {
            this.providerUrl = providerUrl;
            this.notifyListener = notifyListener;
            this.initWith(providerUrl.getEncodedServiceKey() + CONFIGURATORS_SUFFIX);
        }
        private <T> URL overrideUrl(URL providerUrl) {
            return RegistryProtocol.getConfigedInvokerUrl(configurators, providerUrl);
        }
        @Override
        protected void notifyOverrides() {
            notifyListener.doOverrideIfNecessary();
        }
    }
    private class ProviderConfigurationListener extends AbstractConfiguratorListener {