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.

assertEquals("hello1234", url.getPassword());
        assertEquals("host", url.getHost());
        assertEquals(1, url.getPort());
        assertEquals("context/path", url.getPath());
        assertEquals(2, url.getParameters().size());
        assertEquals("1.0.0", url.getParameter("version"));
        assertEquals("morgan", url.getParameter("application"));
        url = url.setPath("path");
        assertEquals("dubbo", url.getProtocol());
        assertEquals("admin", url.getUsername());
        assertEquals("hello1234", url.getPassword());
        assertEquals("host", url.getHost());
        assertEquals(1, url.getPort());
        assertEquals("path", url.getPath());
        assertEquals(2, url.getParameters().size());
        assertEquals("1.0.0", url.getParameter("version"));
        assertEquals("morgan", url.getParameter("application"));
        url = url.setProtocol("protocol");
        assertEquals("protocol", url.getProtocol());
        assertEquals("admin", url.getUsername());
        assertEquals("hello1234", url.getPassword());
        assertEquals("host", url.getHost());
        assertEquals(1, url.getPort());
        assertEquals("path", url.getPath());
        assertEquals(2, url.getParameters().size());
        assertEquals("1.0.0", url.getParameter("version"));
        assertEquals("morgan", url.getParameter("application"));
        url = url.setUsername("username");
        assertEquals("protocol", url.getProtocol());
        assertEquals("username", url.getUsername());