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 ServiceConfig(Service service) {
        appendAnnotation(Service.class, service);
        setMethods(MethodConfig.constructMethodConfig(service.methods()));
    }
    @Deprecated
    private static List<ProtocolConfig> convertProviderToProtocol(List<ProviderConfig> providers) {
        if (CollectionUtils.isEmpty(providers)) {
            return null;
        }
        List<ProtocolConfig> protocols = new ArrayList<ProtocolConfig>(providers.size());
        for (ProviderConfig provider : providers) {
            protocols.add(convertProviderToProtocol(provider));
        }
        return protocols;
    }
    @Deprecated
    private static List<ProviderConfig> convertProtocolToProvider(List<ProtocolConfig> protocols) {
        if (CollectionUtils.isEmpty(protocols)) {
            return null;
        }
        List<ProviderConfig> providers = new ArrayList<ProviderConfig>(protocols.size());
        for (ProtocolConfig provider : protocols) {
            providers.add(convertProtocolToProvider(provider));
        }
        return providers;
    }
    @Deprecated
    private static ProtocolConfig convertProviderToProtocol(ProviderConfig provider) {
        ProtocolConfig protocol = new ProtocolConfig();
        protocol.setName(provider.getProtocol().getName());