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.

this.refresh();
        checkMetadataReport();
        if (StringUtils.isEmpty(interfaceName)) {
            throw new IllegalStateException("<dubbo:service interface=\"\" /> interface not allow null!");
        }
        if (ref instanceof GenericService) {
            interfaceClass = GenericService.class;
            if (StringUtils.isEmpty(generic)) {
                generic = Boolean.TRUE.toString();
            }
        } else {
            try {
                interfaceClass = Class.forName(interfaceName, true, Thread.currentThread()
                        .getContextClassLoader());
            } catch (ClassNotFoundException e) {
                throw new IllegalStateException(e.getMessage(), e);
            }
            checkInterfaceAndMethods(interfaceClass, methods);
            checkRef();
            generic = Boolean.FALSE.toString();
        }
        if (local != null) {
            if ("true".equals(local)) {
                local = interfaceName + "Local";
            }
            Class<?> localClass;
            try {
                localClass = ClassHelper.forNameWithThreadContextClassLoader(local);
            } catch (ClassNotFoundException e) {
                throw new IllegalStateException(e.getMessage(), e);