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.

}
        map.put(Constants.BIND_IP_KEY, hostToBind);
        // 
        String hostToRegistry = getValueFromConfig(protocolConfig, Constants.DUBBO_IP_TO_REGISTRY);
        if (StringUtils.isEmpty(hostToRegistry)) {
            // 
            hostToRegistry = hostToBind;
        }
        map.put(Constants.ANYHOST_KEY, String.valueOf(anyhost));
        return hostToRegistry;
    }
    private String findHostToBindByConnectRegistries(List<URL> registryURLs) {
        if (CollectionUtils.isNotEmpty(registryURLs)) {
            for (URL registryURL : registryURLs) {
                if (Constants.MULTICAST.equalsIgnoreCase(registryURL.getParameter("registry"))) {
                    // 
                    continue;
                }
                try (Socket socket = new Socket()) {
                    SocketAddress addr = new InetSocketAddress(registryURL.getHost(), registryURL.getPort());
                    socket.connect(addr, 1000);
                    return socket.getLocalAddress().getHostAddress();
                } catch (Exception e) {
                    logger.warn(e.getMessage(), e);
                }
            }
        }
        return null;
    }
    /**