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.

* The consumer config (default)
     */
    private ConsumerConfig consumer;
    /**
     * Only the service provider of the specified protocol is invoked, and other protocols are ignored.
     */
    private String protocol;
    /**
     * The interface proxy reference
     */
    private transient volatile T ref;
    /**
     * The invoker of the reference service
     */
    private transient volatile Invoker<?> invoker;
    /**
     * The flag whether the ReferenceConfig has been initialized
     */
    private transient volatile boolean initialized;
    /**
     * whether this ReferenceConfig has been destroyed
     */
    private transient volatile boolean destroyed;
    @SuppressWarnings("unused")
    private final Object finalizerGuardian = new Object() {
        @Override
        protected void finalize() throws Throwable {
            super.finalize();
            if (!ReferenceConfig.this.destroyed) {
                logger.warn("ReferenceConfig(" + url + ") is not DESTROYED when FINALIZE");