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.

setMonitor(module.getMonitor());
            }
        }
        if (application != null) {
            if (registries == null) {
                setRegistries(application.getRegistries());
            }
            if (monitor == null) {
                setMonitor(application.getMonitor());
            }
        }
    }
    private void checkDefault() {
        createProviderIfAbsent();
    }
    private void createProviderIfAbsent() {
        if (provider != null) {
            return;
        }
        setProvider (
                ConfigManager.getInstance()
                        .getDefaultProvider()
                        .orElseGet(() -> {
                            ProviderConfig providerConfig = new ProviderConfig();
                            providerConfig.refresh();
                            return providerConfig;
                        })
        );
    }
    private void checkProtocol() {