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 static final ProxyFactory proxyFactory = ExtensionLoader.getExtensionLoader(ProxyFactory.class).getAdaptiveExtension();
    /**
     * A random port cache, the different protocols who has no port specified have different random port
     */
    private static final Map<String, Integer> RANDOM_PORT_MAP = new HashMap<String, Integer>();
    /**
     * A delayed exposure service timer
     */
    private static final ScheduledExecutorService delayExportExecutor = Executors.newSingleThreadScheduledExecutor(new NamedThreadFactory("DubboServiceDelayExporter", true));
    /**
     * The urls of the services exported
     */
    private final List<URL> urls = new ArrayList<URL>();
    /**
     * The exported services
     */
    private final List<Exporter<?>> exporters = new ArrayList<Exporter<?>>();
    /**
     * The interface name of the exported service
     */
    private String interfaceName;
    /**
     * The interface class of the exported service
     */
    private Class<?> interfaceClass;
    /**
     * The reference of the interface implementation
     */
    private T ref;