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.

protocol.setServer(provider.getServer());
        protocol.setClient(provider.getClient());
        protocol.setCodec(provider.getCodec());
        protocol.setHost(provider.getHost());
        protocol.setPort(provider.getPort());
        protocol.setPath(provider.getPath());
        protocol.setPayload(provider.getPayload());
        protocol.setThreads(provider.getThreads());
        protocol.setParameters(provider.getParameters());
        return protocol;
    }
    @Deprecated
    private static ProviderConfig convertProtocolToProvider(ProtocolConfig protocol) {
        ProviderConfig provider = new ProviderConfig();
        provider.setProtocol(protocol);
        provider.setServer(protocol.getServer());
        provider.setClient(protocol.getClient());
        provider.setCodec(protocol.getCodec());
        provider.setHost(protocol.getHost());
        provider.setPort(protocol.getPort());
        provider.setPath(protocol.getPath());
        provider.setPayload(protocol.getPayload());
        provider.setThreads(protocol.getThreads());
        provider.setParameters(protocol.getParameters());
        return provider;
    }
    private static Integer getRandomPort(String protocol) {
        protocol = protocol.toLowerCase();
        return RANDOM_PORT_MAP.getOrDefault(protocol, Integer.MIN_VALUE);
    }