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.

public String getPath() {
        return path;
    }
    public URL setPath(String path) {
        return new URL(protocol, username, password, host, port, path, getParameters());
    }
    public String getAbsolutePath() {
        if (path != null && !path.startsWith("/")) {
            return "/" + path;
        }
        return path;
    }
    public Map<String, String> getParameters() {
        return parameters;
    }
    public String getParameterAndDecoded(String key) {
        return getParameterAndDecoded(key, null);
    }
    public String getParameterAndDecoded(String key, String defaultValue) {
        return decode(getParameter(key, defaultValue));
    }
    public String getParameter(String key) {
        String value = parameters.get(key);
        if (StringUtils.isEmpty(value)) {
            value = parameters.get(Constants.DEFAULT_KEY_PREFIX + key);
        }
        return value;
    }
    public String getParameter(String key, String defaultValue) {
        String value = getParameter(key);