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.

if (tmpProtocols.stream().noneMatch(prot -> prot.getId().equals(id))) {
                    tmpProtocols.add(ConfigManager.getInstance().getProtocol(id).orElseGet(() -> {
                        ProtocolConfig protocolConfig = new ProtocolConfig();
                        protocolConfig.setId(id);
                        protocolConfig.refresh();
                        return protocolConfig;
                    }));
                }
            });
            if (tmpProtocols.size() > arr.length) {
                throw new IllegalStateException("Too much protocols found, the protocols comply to this service are :" + protocolIds + " but got " + protocols
                        .size() + " registries!");
            }
            setProtocols(tmpProtocols);
        }
    }
    public Class<?> getInterfaceClass() {
        if (interfaceClass != null) {
            return interfaceClass;
        }
        if (ref instanceof GenericService) {
            return GenericService.class;
        }
        try {
            if (interfaceName != null && interfaceName.length() > 0) {
                this.interfaceClass = Class.forName(interfaceName, true, Thread.currentThread()
                        .getContextClassLoader());
            }
        } catch (ClassNotFoundException t) {
            throw new IllegalStateException(t.getMessage(), t);