* [python] Add task decorator for python function

* Add decorator `@task`
* Add a tutorial about it
* Change tutorial doc and combine into traditional docs
  * Add sphinx-inline-tab for better view

* revert not need change

* Correct python function indent

* Correct integration test

Currently the size of our distribute package is up to
800MB, this patch is migrate python gateway server into
api server

The distribute package size before and after this patch is:

```sh
# before
796M   apache-dolphinscheduler-2.0.4-SNAPSHOT-bin.tar.gz

# after
647M   apache-dolphinscheduler-2.0.4-SNAPSHOT-bin.tar.gz
```