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.

public void setProviderIds(String providerIds) {
        this.providerIds = providerIds;
    }
    public String getGeneric() {
        return generic;
    }
    public void setGeneric(String generic) {
        if (StringUtils.isEmpty(generic)) {
            return;
        }
        if (ProtocolUtils.isGeneric(generic)) {
            this.generic = generic;
        } else {
            throw new IllegalArgumentException("Unsupported generic type " + generic);
        }
    }
    @Override
    public void setMock(Boolean mock) {
        throw new IllegalArgumentException("mock doesn't support on provider side");
    }
    @Override
    public void setMock(String mock) {
        throw new IllegalArgumentException("mock doesn't support on provider side");
    }
    public List<URL> getExportedUrls() {
        return urls;
    }
    /**
     * @deprecated Replace to getProtocols()
     */