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 testInterface1() throws Exception {
        Assertions.assertThrows(IllegalStateException.class, () -> {
            ServiceConfig<DemoService> service = new ServiceConfig<DemoService>();
            service.setInterface(DemoServiceImpl.class);
        });
    }
    @Test
    public void testInterface2() throws Exception {
        ServiceConfig<DemoService> service = new ServiceConfig<DemoService>();
        service.setInterface(DemoService.class);
        assertThat(service.getInterface(), equalTo(DemoService.class.getName()));
    }
    @Test
    public void testProvider() throws Exception {
        ServiceConfig service = new ServiceConfig();
        ProviderConfig provider = new ProviderConfig();
        service.setProvider(provider);
        assertThat(service.getProvider(), is(provider));
    }
    @Test
    public void testGeneric1() throws Exception {
        ServiceConfig service = new ServiceConfig();
        service.setGeneric(GENERIC_SERIALIZATION_DEFAULT);
        assertThat(service.getGeneric(), equalTo(GENERIC_SERIALIZATION_DEFAULT));
        service.setGeneric(GENERIC_SERIALIZATION_NATIVE_JAVA);
        assertThat(service.getGeneric(), equalTo(GENERIC_SERIALIZATION_NATIVE_JAVA));
        service.setGeneric(GENERIC_SERIALIZATION_BEAN);
        assertThat(service.getGeneric(), equalTo(GENERIC_SERIALIZATION_BEAN));
    }
    @Test