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.

if (Constants.PROTOCOL_KEY.equals(key)) {
            return protocol;
        }
        if (Constants.USERNAME_KEY.equals(key)) {
            return username;
        }
        if (Constants.PASSWORD_KEY.equals(key)) {
            return password;
        }
        if (Constants.HOST_KEY.equals(key)) {
            return host;
        }
        if (Constants.PORT_KEY.equals(key)) {
            return String.valueOf(port);
        }
        if (Constants.PATH_KEY.equals(key)) {
            return path;
        }
        return getParameter(key);
    }
    public Map<String, String> toMap() {
        Map<String, String> map = new HashMap<>(parameters);
        if (protocol != null) {
            map.put(Constants.PROTOCOL_KEY, protocol);
        }
        if (username != null) {
            map.put(Constants.USERNAME_KEY, username);
        }
        if (password != null) {
            map.put(Constants.USERNAME_KEY, password);