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.

import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.ExecutorService;
import static java.util.concurrent.Executors.newSingleThreadExecutor;
import static org.apache.dubbo.common.Constants.ACCEPT_FOREIGN_IP;
import static org.apache.dubbo.common.Constants.ANY_VALUE;
import static org.apache.dubbo.common.Constants.BIND_IP_KEY;
import static org.apache.dubbo.common.Constants.BIND_PORT_KEY;
import static org.apache.dubbo.common.Constants.CATEGORY_KEY;
import static org.apache.dubbo.common.Constants.CHECK_KEY;
import static org.apache.dubbo.common.Constants.COMMA_SPLIT_PATTERN;
import static org.apache.dubbo.common.Constants.CONFIGURATORS_CATEGORY;
import static org.apache.dubbo.common.Constants.CONFIGURATORS_SUFFIX;
import static org.apache.dubbo.common.Constants.CONSUMERS_CATEGORY;
import static org.apache.dubbo.common.Constants.CONSUMER_PROTOCOL;
import static org.apache.dubbo.common.Constants.DEFAULT_DIRECTORY;
import static org.apache.dubbo.common.Constants.DEFAULT_REGISTER_CONSUMER_KEYS;
import static org.apache.dubbo.common.Constants.DEFAULT_REGISTER_PROVIDER_KEYS;
import static org.apache.dubbo.common.Constants.DEFAULT_REGISTRY;
import static org.apache.dubbo.common.Constants.EXPORT_KEY;
import static org.apache.dubbo.common.Constants.EXTRA_KEYS_KEY;
import static org.apache.dubbo.common.Constants.HIDE_KEY_PREFIX;
import static org.apache.dubbo.common.Constants.INTERFACES;
import static org.apache.dubbo.common.Constants.METHODS_KEY;
import static org.apache.dubbo.common.Constants.MONITOR_KEY;
import static org.apache.dubbo.common.Constants.OVERRIDE_PROTOCOL;
import static org.apache.dubbo.common.Constants.PROVIDERS_CATEGORY;
import static org.apache.dubbo.common.Constants.PROVIDER_PROTOCOL;
import static org.apache.dubbo.common.Constants.QOS_ENABLE;