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 (StringUtils.isEmpty(value)) {
            return defaultValue;
        }
        return value;
    }
    public String[] getParameter(String key, String[] defaultValue) {
        String value = getParameter(key);
        if (StringUtils.isEmpty(value)) {
            return defaultValue;
        }
        return Constants.COMMA_SPLIT_PATTERN.split(value);
    }
    public List<String> getParameter(String key, List<String> defaultValue) {
        String value = getParameter(key);
        if (value == null || value.length() == 0) {
            return defaultValue;
        }
        String[] strArray = Constants.COMMA_SPLIT_PATTERN.split(value);
        return Arrays.asList(strArray);
    }
    private Map<String, Number> getNumbers() {
        if (numbers == null) { 
            numbers = new ConcurrentHashMap<>();
        }
        return numbers;
    }
    private Map<String, URL> getUrls() {
        if (urls == null) { 
            urls = new ConcurrentHashMap<>();
        }