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 retryKey = method.getName() + ".retry";
                if (map.containsKey(retryKey)) {
                    String retryValue = map.remove(retryKey);
                    if ("false".equals(retryValue)) {
                        map.put(method.getName() + ".retries", "0");
                    }
                }
                List<ArgumentConfig> arguments = method.getArguments();
                if (CollectionUtils.isNotEmpty(arguments)) {
                    for (ArgumentConfig argument : arguments) {
                        // 
                        if (argument.getType() != null && argument.getType().length() > 0) {
                            Method[] methods = interfaceClass.getMethods();
                            // 
                            if (methods != null && methods.length > 0) {
                                for (int i = 0; i < methods.length; i++) {
                                    String methodName = methods[i].getName();
                                    // 
                                    if (methodName.equals(method.getName())) {
                                        Class<?>[] argtypes = methods[i].getParameterTypes();
                                        // 
                                        if (argument.getIndex() != -1) {
                                            if (argtypes[argument.getIndex()].getName().equals(argument.getType())) {
                                                appendParameters(map, argument, method.getName() + "." + argument.getIndex());
                                            } else {
                                                throw new IllegalArgumentException("Argument config error : the index attribute and type attribute not match :index :" + argument.getIndex() + ", type:" + argument.getType());
                                            }
                                        } else {
                                            // 
                                            for (int j = 0; j < argtypes.length; j++) {