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.

* url example:
 * <ul>
 * <li>http://www.facebook.com/friends?param1=value1&amp;param2=value2
 * <li>http://username:password@10.20.130.230:8080/list?version=1.0.0
 * <li>ftp://username:password@192.168.1.7:21/1/read.txt
 * <li>registry://192.168.1.7:9090/org.apache.dubbo.service1?param1=value1&amp;param2=value2
 * </ul>
 * <p>
 * Some strange example below:
 * <ul>
 * <li>192.168.1.3:20880<br>
 * for this case, url protocol = null, url host = 192.168.1.3, port = 20880, url path = null
 * <li>file:///home/user1/router.js?type=script<br>
 * for this case, url protocol = file, url host = null, url path = home/user1/router.js
 * <li>file://home/user1/router.js?type=script<br>
 * for this case, url protocol = file, url host = home, url path = user1/router.js
 * <li>file:///D:/1/router.js?type=script<br>
 * for this case, url protocol = file, url host = null, url path = D:/1/router.js
 * <li>file:/D:/1/router.js?type=script<br>
 * same as above file:///D:/1/router.js?type=script
 * <li>/home/user1/router.js?type=script <br>
 * for this case, url protocol = null, url host = null, url path = home/user1/router.js
 * <li>home/user1/router.js?type=script <br>
 * for this case, url protocol = null, url host = home, url path = user1/router.js
 * </ul>
 *
 * @see java.net.URL
 * @see java.net.URI
 */
public