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.

public ProviderConfigurationListener() {
            this.initWith(ApplicationModel.getApplication() + CONFIGURATORS_SUFFIX);
        }
        /**
         * Get existing configuration rule and override provider url before exporting.
         *
         * @param providerUrl
         * @param <T>
         * @return
         */
        private <T> URL overrideUrl(URL providerUrl) {
            return RegistryProtocol.getConfigedInvokerUrl(configurators, providerUrl);
        }
        @Override
        protected void notifyOverrides() {
            overrideListeners.values().forEach(listener -> ((OverrideListener) listener).doOverrideIfNecessary());
        }
    }
    /**
     * exporter proxy, establish the corresponding relationship between the returned exporter and the exporter
     * exported by the protocol, and can modify the relationship at the time of override.
     *
     * @param <T>
     */
    private class ExporterChangeableWrapper<T> implements Exporter<T> {
        private final ExecutorService executor = newSingleThreadExecutor(new NamedThreadFactory("Exporter-Unexport", true));