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.

buf.append("?");
                        }
                        first = false;
                    } else {
                        buf.append("&");
                    }
                    buf.append(entry.getKey());
                    buf.append("=");
                    buf.append(entry.getValue() == null ? "" : entry.getValue().trim());
                }
            }
        }
    }
    private String buildString(boolean appendUser, boolean appendParameter, String... parameters) {
        return buildString(appendUser, appendParameter, false, false, parameters);
    }
    private String buildString(boolean appendUser, boolean appendParameter, boolean useIP, boolean useService, String... parameters) {
        StringBuilder buf = new StringBuilder();
        if (StringUtils.isNotEmpty(protocol)) {
            buf.append(protocol);
            buf.append("://");
        }
        if (appendUser && StringUtils.isNotEmpty(username)) {
            buf.append(username);
            if (password != null && password.length() > 0) {
                buf.append(":");
                buf.append(password);
            }
            buf.append("@");
        }