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 final int HTTPCLIENTCONNECTIONMANAGER_MAXTOTAL = 20;
    private static final int HTTPCLIENT_KEEPALIVEDURATION = 30 * 1000;
    private static final int HTTPCLIENTCONNECTIONMANAGER_CLOSEWAITTIME_MS = 1000;
    private static final int HTTPCLIENTCONNECTIONMANAGER_CLOSEIDLETIME_S = 30;
    private final Map<String, RestServer> servers = new ConcurrentHashMap<>();
    private final RestServerFactory serverFactory = new RestServerFactory();
    
    private final List<ResteasyClient> clients = Collections.synchronizedList(new LinkedList<>());
    private volatile ConnectionMonitor connectionMonitor;
    public RestProtocol() {
        super(WebApplicationException.class, ProcessingException.class);
    }
    public void setHttpBinder(HttpBinder httpBinder) {
        serverFactory.setHttpBinder(httpBinder);
    }
    @Override
    public int getDefaultPort() {
        return DEFAULT_PORT;
    }
    @Override
    protected <T> Runnable doExport(T impl, Class<T> type, URL url) throws RpcException {
        String addr = getAddr(url);
        Class implClass = ApplicationModel.getProviderModel(url.getServiceKey()).getServiceInstance().getClass();
        RestServer server = servers.computeIfAbsent(addr, restServer -> {
            RestServer s = serverFactory.createServer(url.getParameter(Constants.SERVER_KEY, DEFAULT_SERVER));
            s.start(url);
            return s;
        });
        String contextPath = getContextPath(url);
        if ("servlet".equalsIgnoreCase(url.getParameter(Constants.SERVER_KEY, DEFAULT_SERVER))) {