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 (version != null && version.length() > 0) {
            buf.append(":").append(version);
        }
        return buf.toString();
    }
    public String toServiceStringWithoutResolving() {
        return buildString(true, false, false, true);
    }
    public String toServiceString() {
        return buildString(true, false, true, true);
    }
    @Deprecated
    public String getServiceName() {
        return getServiceInterface();
    }
    public String getServiceInterface() {
        return getParameter(Constants.INTERFACE_KEY, path);
    }
    public URL setServiceInterface(String service) {
        return addParameter(Constants.INTERFACE_KEY, service);
    }
    /**
     * @see #getParameter(String, int)
     * @deprecated Replace to <code>getParameter(String, int)</code>
     */
    @Deprecated
    public int getIntParameter(String key) {
        return getParameter(key, 0);
    }
    /**