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 (ArrayUtils.isNotEmpty(backups)) {
            for (String backup : backups) {
                address.append(",");
                address.append(appendDefaultPort(backup, defaultPort));
            }
        }
        return address.toString();
    }
    public List<URL> getBackupUrls() {
        List<URL> urls = new ArrayList<>();
        urls.add(this);
        String[] backups = getParameter(Constants.BACKUP_KEY, new String[0]);
        if (backups != null && backups.length > 0) {
            for (String backup : backups) {
                urls.add(this.setAddress(backup));
            }
        }
        return urls;
    }
    static String appendDefaultPort(String address, int defaultPort) {
        if (address != null && address.length() > 0 && defaultPort > 0) {
            int i = address.indexOf(':');
            if (i < 0) {
                return address + ":" + defaultPort;
            } else if (Integer.parseInt(address.substring(i + 1)) == 0) {
                return address.substring(0, i + 1) + defaultPort;
            }
        }
        return address;
    }