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.

class URL implements Serializable {
    private static final long serialVersionUID = -1985165475234910535L;
    private final String protocol;
    private final String username;
    private final String password;
    
    private final String host;
    
    private final int port;
    private final String path;
    private final Map<String, String> parameters;
    
    private volatile transient Map<String, Number> numbers;
    private volatile transient Map<String, URL> urls;
    private volatile transient String ip;
    private volatile transient String full;
    private volatile transient String identity;
    private volatile transient String parameter;
    private volatile transient String string;
    protected URL() {
        this.protocol = null;
        this.username = null;
        this.password = null;