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.

return new java.net.URL(toString());
        } catch (MalformedURLException e) {
            throw new IllegalStateException(e.getMessage(), e);
        }
    }
    public InetSocketAddress toInetSocketAddress() {
        return new InetSocketAddress(host, port);
    }
    /**
     * The format is '{group}/{interfaceName/path}*{version}'
     *
     * @return
     */
    public String getEncodedServiceKey() {
        String serviceKey = this.getServiceKey();
        serviceKey = serviceKey.replaceFirst("/", "*");
        return serviceKey;
    }
    public String getServiceKey() {
        String inf = getServiceInterface();
        if (inf == null) {
            return null;
        }
        StringBuilder buf = new StringBuilder();
        String group = getParameter(Constants.GROUP_KEY);
        if (group != null && group.length() > 0) {
            buf.append(group).append("/");
        }
        buf.append(inf);
        String version = getParameter(Constants.VERSION_KEY);