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.

private static void putRandomPort(String protocol, Integer port) {
        protocol = protocol.toLowerCase();
        if (!RANDOM_PORT_MAP.containsKey(protocol)) {
            RANDOM_PORT_MAP.put(protocol, port);
            logger.warn("Use random available port(" + port + ") for protocol " + protocol);
        }
    }
    public URL toUrl() {
        return urls.isEmpty() ? null : urls.iterator().next();
    }
    public List<URL> toUrls() {
        return urls;
    }
    @Parameter(excluded = true)
    public boolean isExported() {
        return exported;
    }
    @Parameter(excluded = true)
    public boolean isUnexported() {
        return unexported;
    }
    public void checkAndUpdateSubConfigs() {
        // 
        completeCompoundConfigs();
        // 
        startConfigCenter();
        checkDefault();
        checkApplication();
        checkRegistry();
        checkProtocol();