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.

public void setRef(T ref) {
        this.ref = ref;
    }
    @Parameter(excluded = true)
    public String getPath() {
        return path;
    }
    public void setPath(String path) {
        checkPathName(Constants.PATH_KEY, path);
        this.path = path;
    }
    public List<MethodConfig> getMethods() {
        return methods;
    }
    // 
    @SuppressWarnings("unchecked")
    public void setMethods(List<? extends MethodConfig> methods) {
        this.methods = (List<MethodConfig>) methods;
    }
    public ProviderConfig getProvider() {
        return provider;
    }
    public void setProvider(ProviderConfig provider) {
        ConfigManager.getInstance().addProvider(provider);
        this.provider = provider;
    }
    @Parameter(excluded = true)
    public String getProviderIds() {
        return providerIds;
    }