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.

String host;
        if (useIP) {
            host = getIp();
        } else {
            host = getHost();
        }
        if (host != null && host.length() > 0) {
            buf.append(host);
            if (port > 0) {
                buf.append(":");
                buf.append(port);
            }
        }
        String path;
        if (useService) {
            path = getServiceKey();
        } else {
            path = getPath();
        }
        if (path != null && path.length() > 0) {
            buf.append("/");
            buf.append(path);
        }
        if (appendParameter) {
            buildParameters(buf, true, parameters);
        }
        return buf.toString();
    }
    public java.net.URL toJavaURL() {
        try {