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.

this.host = null;
        this.port = 0;
        this.path = null;
        this.parameters = null;
    }
    public URL(String protocol, String host, int port) {
        this(protocol, null, null, host, port, null, (Map<String, String>) null);
    }
    public URL(String protocol, String host, int port, String[] pairs) { 
        this(protocol, null, null, host, port, null, CollectionUtils.toStringMap(pairs));
    }
    public URL(String protocol, String host, int port, Map<String, String> parameters) {
        this(protocol, null, null, host, port, null, parameters);
    }
    public URL(String protocol, String host, int port, String path) {
        this(protocol, null, null, host, port, path, (Map<String, String>) null);
    }
    public URL(String protocol, String host, int port, String path, String... pairs) {
        this(protocol, null, null, host, port, path, CollectionUtils.toStringMap(pairs));
    }
    public URL(String protocol, String host, int port, String path, Map<String, String> parameters) {
        this(protocol, null, null, host, port, path, parameters);
    }
    public URL(String protocol, String username, String password, String host, int port, String path) {
        this(protocol, username, password, host, port, path, (Map<String, String>) null);
    }
    public URL(String protocol, String username, String password, String host, int port, String path, String... pairs) {
        this(protocol, username, password, host, port, path, CollectionUtils.toStringMap(pairs));
    }
    public URL(String protocol, String username, String password, String host, int port, String path, Map<String, String> parameters) {