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.

}
        // 
        map.put(Constants.BIND_PORT_KEY, String.valueOf(portToBind));
        // 
        String portToRegistryStr = getValueFromConfig(protocolConfig, Constants.DUBBO_PORT_TO_REGISTRY);
        Integer portToRegistry = parsePort(portToRegistryStr);
        if (portToRegistry == null) {
            portToRegistry = portToBind;
        }
        return portToRegistry;
    }
    private Integer parsePort(String configPort) {
        Integer port = null;
        if (configPort != null && configPort.length() > 0) {
            try {
                Integer intPort = Integer.parseInt(configPort);
                if (isInvalidPort(intPort)) {
                    throw new IllegalArgumentException("Specified invalid port from env value:" + configPort);
                }
                port = intPort;
            } catch (Exception e) {
                throw new IllegalArgumentException("Specified invalid port from env value:" + configPort);
            }
        }
        return port;
    }
    private String getValueFromConfig(ProtocolConfig protocolConfig, String key) {
        String protocolPrefix = protocolConfig.getName().toUpperCase() + "_";
        String port = ConfigUtils.getSystemProperty(protocolPrefix + key);
        if (StringUtils.isEmpty(port)) {