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.

}
    protected Class getServiceClass(T ref) {
        return ref.getClass();
    }
    /**
     * Register & bind IP address for service provider, can be configured separately.
     * Configuration priority: environment variables -> java system properties -> host property in config file ->
     * /etc/hosts -> default network address -> first available network address
     *
     * @param protocolConfig
     * @param registryURLs
     * @param map
     * @return
     */
    private String findConfigedHosts(ProtocolConfig protocolConfig, List<URL> registryURLs, Map<String, String> map) {
        boolean anyhost = false;
        String hostToBind = getValueFromConfig(protocolConfig, Constants.DUBBO_IP_TO_BIND);
        // 
        if (StringUtils.isEmpty(hostToBind)) {
            hostToBind = protocolConfig.getHost();
            if (provider != null && StringUtils.isEmpty(hostToBind)) {
                hostToBind = provider.getHost();
            }
            if (StringUtils.isEmpty(hostToBind)) {
                anyhost = true;
                hostToBind = getLocalHost();
                if (StringUtils.isEmpty(hostToBind)) {
                    hostToBind = findHostToBindByConnectRegistries(registryURLs);
                }
            }