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.

logger.info("Refer dubbo service " + interfaceClass.getName() + " from url " + invoker.getUrl());
        }
        /**
         * @since 2.7.0
         * ServiceData Store
         */
        MetadataReportService metadataReportService = null;
        if ((metadataReportService = getMetadataReportService()) != null) {
            URL consumerURL = new URL(Constants.CONSUMER_PROTOCOL, map.remove(Constants.REGISTER_IP_KEY), 0, map.get(Constants.INTERFACE_KEY), map);
            metadataReportService.publishConsumer(consumerURL);
        }
        // 
        return (T) proxyFactory.getProxy(invoker);
    }
    /**
     * Figure out should refer the service in the same JVM from configurations. The default behavior is true
     * 1. if injvm is specified, then use it
     * 2. then if a url is specified, then assume it's a remote call
     * 3. otherwise, check scope parameter
     * 4. if scope is not specified but the target service is provided in the same JVM, then prefer to make the local
     * call, which is the default behavior
     */
    protected boolean shouldJvmRefer(Map<String, String> map) {
        URL tmpUrl = new URL("temp", "localhost", 0, map);
        boolean isJvmRefer;
        if (isInjvm() == null) {
            // 
            if (url != null && url.length() > 0) {
                isJvmRefer = false;
            } else {