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 testUniqueServiceName() throws Exception {
        ServiceBuilder<Greeting> builder = new ServiceBuilder<>();
        builder.group("dubbo").interfaceClass(Greeting.class).version("1.0.0");
        ServiceConfig<Greeting> service = builder.build();
        assertThat(service.getUniqueServiceName(), equalTo("dubbo/" + Greeting.class.getName() + ":1.0.0"));
    }
    @Test
    void path() {
        ServiceBuilder builder = new ServiceBuilder();
        builder.path("path");
        Assertions.assertEquals("path", builder.build().getPath());
    }
    @Test
    void addMethod() {
        MethodConfig method = new MethodConfig();
        ServiceBuilder builder = new ServiceBuilder();
        builder.addMethod(method);
        Assertions.assertTrue(builder.build().getMethods().contains(method));
        Assertions.assertEquals(1, builder.build().getMethods().size());
    }
    @Test
    void addMethods() {
        MethodConfig method = new MethodConfig();
        ServiceBuilder builder = new ServiceBuilder();
        builder.addMethods(Collections.singletonList(method));
        Assertions.assertTrue(builder.build().getMethods().contains(method));
        Assertions.assertEquals(1, builder.build().getMethods().size());
    }
    @Test
    void provider() {