Allow passing in traitlets via commandline

[yuvipanda]

Without this, you always needed a repo2docker_config.py
file to configure anything. This PR makes r2d match the
behavior of most traitlets based applications (like jupyter_server,
jupyterhub, nbconvert, etc)

Fixes #1112

[manics]

Do you know how to get the --help-all argument working? E.g. for jupyterhub

$ jupyterhub --help
...
Options
=======
The options below are convenience aliases to configurable class-options,
as listed in the "Equivalent to" description-line of the aliases.
To see all configurable class-options for some <cmd>, use:
    <cmd> --help-all

...

$ jupyterhub --help-all
...
Class options
=============
The command-line option below sets the respective configurable class-parameter:
    --Class.parameter=value
This line is evaluated in Python, so simple expressions are allowed.
For instance, to set `C.a=[0,1,2]`, you may type this:
    --C.a='range(3)'

Application(SingletonConfigurable) options
------------------------------------------
--Application.log_datefmt=<Unicode>
    The date format used by logging formatters for %(asctime)s
    Default: '%Y-%m-%d %H:%M:%S'

...

[yuvipanda]

@manics so we're mixing argparse and traitlets here, which is causing some of these issues. Ideally we would just be using traitlets and this will go away... I'm not entirely sure how to get --help-all working, other than to try get rid of argparse.

[manics]

@minrk Do you know of an easy fix? If not let's merge!

[minrk]

I added --help-all. Application().print_help() is the method to call (classes=True is --help-all).

It does get a little funky, since there are two full argparse CLI instances running now (the one generated by traitlets, and our handcrafted one). I think if we want to have full traitlets CLI parsing, perhaps we should use traitlets CLI parsing. I understand not wanting to do that (it's simpler and nicer to write your own parser), but if we want the --Class.trait=value magic, it might be better to go all-in. We ought to get things like --show-config and friends for free, too.

[manics]

CI failure is unrelated

Traceback (most recent call last):
  File "/usr/local/bin/jupyter-notebook", line 5, in <module>
    from notebook.notebookapp import main
  File "/usr/local/lib/python3.5/site-packages/notebook/notebookapp.py", line 134, in <module>
    from .terminal import TerminalManager
  File "/usr/local/lib/python3.5/site-packages/notebook/terminal/__init__.py", line 4, in <module>
    import terminado
  File "/usr/local/lib/python3.5/site-packages/terminado/__init__.py", line 7, in <module>
    from .websocket import TermSocket
  File "/usr/local/lib/python3.5/site-packages/terminado/websocket.py", line 87
    self.log_terminal_output(f'STDOUT: {content[1]}')

Also occurred on main this morning 😞 https://github.com/jupyterhub/repo2docker/runs/4963525952?check_suite_focus=true

[yuvipanda]

@minrk agree going all inn is probably a good idea. Do you want me to do that to move this PR forward? Or can / should it be deferred?

[minrk]

Or can / should it be deferred?

It's fine to defer it. The biggest downside right now is that the --help-all output is a little confusing since two parsers each think they are displaying all the options, so the flow is a bit awkward.

CI failure is unrelated

The test failure is a metadata bug in terminado 0.13. Should be fixed by jupyter/terminado#140 once 0.13.0 is yanked.

[manics]

Green! 🚀

Thanks all!

[manics]

Follow-up issue here: #1129