Add new-style 2.0 command names for Airflow 1.10.x (#12725)

This introduces the new style commands for 1.10, but keeps all the
existing commands working, they just don't show in the top level
`airflow --help` anymore.

This was done by taking the argparse groups/sub-commands from master
(along with supporting functions) and copying it here, then providing
needed shims for compat.

I have not added any new sub-commands, nor changed any existing
behaviours, so the 1.10 commands won't change in behaviour.

UPDATING.md

@@ -25,6 +25,7 @@ assists users migrating to a new version.
 <!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
 **Table of contents**
 
+- [Airflow 1.10.14](#airflow-11014)
 - [Airflow 1.10.13](#airflow-11013)
 - [Airflow 1.10.12](#airflow-11012)
 - [Airflow 1.10.11](#airflow-11011)
@@ -61,6 +62,62 @@ More tips can be found in the guide:
 https://developers.google.com/style/inclusive-documentation
 
 -->
+## Airflow 1.10.14
+
+### Airflow CLI changes in line with  2.0
+
+The Airflow CLI has been organized so that related commands are grouped together as subcommands,
+which means that if you use these commands in your scripts, you have to make changes to them.
+
+This section describes the changes that have been made, and what you need to do to update your script.
+
+The ability to manipulate users from the command line has been changed. ``airflow create_user``,  ``airflow delete_user``
+ and ``airflow list_users`` has been grouped to a single command `airflow users` with optional flags `create`, `list` and `delete`.
+
+The `airflow list_dags` command is now `airflow dags list`, `airflow pause` is `airflow dags pause`, etc.
+
+In Airflow 1.10 and 2.0 there is an `airflow config` command but there is a difference in behavior. In Airflow 1.10,
+it prints all config options while in Airflow 2.0, it's a command group. `airflow config` is now `airflow config list`.
+You can check other options by running the command `airflow config --help`
+
+Compatibility with the old CLI has been maintained, but they will no longer appear in the help
+
+You can learn about the commands by running ``airflow --help``. For example to get help about the ``celery`` group command,
+you have to run the help command: ``airflow celery --help``.
+
+| Old command                   | New command                        |     Group          |
+|-------------------------------|------------------------------------|--------------------|
+| ``airflow worker``            | ``airflow celery worker``          |    ``celery``      |
+| ``airflow flower``            | ``airflow celery flower``          |    ``celery``      |
+| ``airflow trigger_dag``       | ``airflow dags trigger``           |    ``dags``        |
+| ``airflow delete_dag``        | ``airflow dags delete``            |    ``dags``        |
+| ``airflow show_dag``          | ``airflow dags show``              |    ``dags``        |
+| ``airflow list_dag``          | ``airflow dags list``              |    ``dags``        |
+| ``airflow dag_status``        | ``airflow dags status``            |    ``dags``        |
+| ``airflow backfill``          | ``airflow dags backfill``          |    ``dags``        |
+| ``airflow list_dag_runs``     | ``airflow dags list-runs``         |    ``dags``        |
+| ``airflow pause``             | ``airflow dags pause``             |    ``dags``        |
+| ``airflow unpause``           | ``airflow dags unpause``           |    ``dags``        |
+| ``airflow next_execution``    | ``airflow dags next-execution``    |    ``dags``        |
+| ``airflow test``              | ``airflow tasks test``             |    ``tasks``       |
+| ``airflow clear``             | ``airflow tasks clear``            |    ``tasks``       |
+| ``airflow list_tasks``        | ``airflow tasks list``             |    ``tasks``       |
+| ``airflow task_failed_deps``  | ``airflow tasks failed-deps``      |    ``tasks``       |
+| ``airflow task_state``        | ``airflow tasks state``            |    ``tasks``       |
+| ``airflow run``               | ``airflow tasks run``              |    ``tasks``       |
+| ``airflow render``            | ``airflow tasks render``           |    ``tasks``       |
+| ``airflow initdb``            | ``airflow db init``                |     ``db``         |
+| ``airflow resetdb``           | ``airflow db reset``               |     ``db``         |
+| ``airflow upgradedb``         | ``airflow db upgrade``             |     ``db``         |
+| ``airflow checkdb``           | ``airflow db check``               |     ``db``         |
+| ``airflow shell``             | ``airflow db shell``               |     ``db``         |
+| ``airflow pool``              | ``airflow pools``                  |     ``pools``      |
+| ``airflow create_user``       | ``airflow users create``           |     ``users``      |
+| ``airflow delete_user``       | ``airflow users delete``           |     ``users``      |
+| ``airflow list_users``        | ``airflow users list``             |     ``users``      |
+| ``airflow rotate_fernet_key`` | ``airflow rotate-fernet-key``      |                    |
+| ``airflow sync_perm``         | ``airflow sync-perm``              |                    |
+
 ## Airflow 1.10.13
 
 ### TimeSensor is now timezone aware