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.

String prefix = method + ".";
            for (String fullKey : parameters.keySet()) {
                if (fullKey.startsWith(prefix)) {
                    return true;
                }
            }
            return false;
        }
        String value = getMethodParameter(method, key);
        return value != null && value.length() > 0;
    }
    public boolean isLocalHost() {
        return NetUtils.isLocalHost(host) || getParameter(Constants.LOCALHOST_KEY, false);
    }
    public boolean isAnyHost() {
        return Constants.ANYHOST_VALUE.equals(host) || getParameter(Constants.ANYHOST_KEY, false);
    }
    public URL addParameterAndEncoded(String key, String value) {
        if (StringUtils.isEmpty(value)) {
            return this;
        }
        return addParameter(key, encode(value));
    }
    public URL addParameter(String key, boolean value) {
        return addParameter(key, String.valueOf(value));
    }
    public URL addParameter(String key, char value) {
        return addParameter(key, String.valueOf(value));
    }
    public URL addParameter(String key, byte value) {