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 (methods.length == 0) {
                logger.warn("No method found in service interface " + interfaceClass.getName());
                map.put(Constants.METHODS_KEY, Constants.ANY_VALUE);
            } else {
                map.put(Constants.METHODS_KEY, StringUtils.join(new HashSet<String>(Arrays.asList(methods)), ","));
            }
        }
        if (!ConfigUtils.isEmpty(token)) {
            if (ConfigUtils.isDefault(token)) {
                map.put(Constants.TOKEN_KEY, UUID.randomUUID().toString());
            } else {
                map.put(Constants.TOKEN_KEY, token);
            }
        }
        // 
        String contextPath = protocolConfig.getContextpath();
        if (StringUtils.isEmpty(contextPath) && provider != null) {
            contextPath = provider.getContextpath();
        }
        String host = this.findConfigedHosts(protocolConfig, registryURLs, map);
        Integer port = this.findConfigedPorts(protocolConfig, name, map);
        URL url = new URL(name, host, port, (StringUtils.isEmpty(contextPath) ? "" : contextPath + "/") + path, map);
        if (ExtensionLoader.getExtensionLoader(ConfiguratorFactory.class)
                .hasExtension(url.getProtocol())) {
            url = ExtensionLoader.getExtensionLoader(ConfiguratorFactory.class)
                    .getExtension(url.getProtocol()).getConfigurator(url).configure(url);
        }
        String scope = url.getParameter(Constants.SCOPE_KEY);
        // 
        if (!Constants.SCOPE_NONE.equalsIgnoreCase(scope)) {