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.

}
        if (host != null) {
            map.put(Constants.HOST_KEY, host);
        }
        if (port > 0) {
            map.put(Constants.PORT_KEY, String.valueOf(port));
        }
        if (path != null) {
            map.put(Constants.PATH_KEY, path);
        }
        return map;
    }
    @Override
    public String toString() {
        if (string != null) {
            return string;
        }
        return string = buildString(false, true); 
    }
    public String toString(String... parameters) {
        return buildString(false, true, parameters); 
    }
    public String toIdentityString() {
        if (identity != null) {
            return identity;
        }
        return identity = buildString(true, false); 
    }
    public String toIdentityString(String... parameters) {
        return buildString(true, false, parameters);