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.

private void checkDefault() {
        createConsumerIfAbsent();
    }
    private void createConsumerIfAbsent() {
        if (consumer != null) {
            return;
        }
        setConsumer(
                        ConfigManager.getInstance()
                            .getDefaultConsumer()
                            .orElseGet(() -> {
                                ConsumerConfig consumerConfig = new ConsumerConfig();
                                consumerConfig.refresh();
                                return consumerConfig;
                            })
                );
    }
    private void completeCompoundConfigs() {
        if (consumer != null) {
            if (application == null) {
                setApplication(consumer.getApplication());
            }
            if (module == null) {
                setModule(consumer.getModule());
            }
            if (registries == null) {
                setRegistries(consumer.getRegistries());
            }
            if (monitor == null) {
                setMonitor(consumer.getMonitor());