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.

}
        return ref;
    }
    public synchronized void destroy() {
        if (ref == null) {
            return;
        }
        if (destroyed) {
            return;
        }
        destroyed = true;
        try {
            invoker.destroy();
        } catch (Throwable t) {
            logger.warn("Unexpected error occured when destroy invoker of ReferenceConfig(" + url + ").", t);
        }
        invoker = null;
        ref = null;
    }
    private void init() {
        if (initialized) {
            return;
        }
        initialized = true;
        checkStubAndLocal(interfaceClass);
        checkMock(interfaceClass);
        Map<String, String> map = new HashMap<String, String>();
        map.put(Constants.SIDE_KEY, Constants.CONSUMER_SIDE);
        appendRuntimeParameters(map);
        if (!isGeneric()) {