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.

}
        for (Map.Entry<String, RestServer> entry : servers.entrySet()) {
            try {
                if (logger.isInfoEnabled()) {
                    logger.info("Closing the rest server at " + entry.getKey());
                }
                entry.getValue().stop();
            } catch (Throwable t) {
                logger.warn("Error closing rest server", t);
            }
        }
        servers.clear();
        if (logger.isInfoEnabled()) {
            logger.info("Closing rest clients");
        }
        for (ResteasyClient client : clients) {
            try {
                client.close();
            } catch (Throwable t) {
                logger.warn("Error closing rest client", t);
            }
        }
        clients.clear();
    }
    protected String getContextPath(URL url) {
        String contextPath = url.getPath();
        return contextPath.endsWith("/") ? contextPath.substring(0, contextPath.length() - 1) : contextPath;
    }
    protected class ConnectionMonitor extends Thread {
        private volatile boolean shutdown;