Set default log level via config

[riga]

Description

This PR allows for the configuration of the default log level, which is DEBUG by default.

Motivation and Context

It might be a first world problem, but sometimes it would be nice to control the default log level without the need to create (and deploy) a logging config file, but rather by a simple environment variable. The value of this variable must match the name of a python logging level. Usage example:

LUIGI_DEFAULT_LOG_LEVEL=INFO luigi --module my_module MyTask --x 123

An other option would be to add default_log_level to the core config and to prefer the environment variable if set (at least that's what most libs do).

Tests

Too trivial to test :P

Docs

I couldn't find a place in the docs for this feature, but I sure can add a sentence or two to a section of your choice.

[mention-bot]

@riga, thanks for your PR! By analyzing the history of the files in this pull request, we identified @dlstadther, @themalkolm and @Tarrasch to be potential reviewers.

[Tarrasch]

Is there any reason you don't set this using the core config class? Then it could be automatically documented through the --help flag. Also, a ChoiceParameter would check that you don't misspell the logging levels.

Would you consider to refactor into such an implementation?

[Tarrasch]

Overall awesome patch. I have wanted this too sometimes. :)

Oh, bonus points if you assert that if there's a config set to read the logging-config, then luigi shouldn't allow one to set this new config variable as it will not take affect.

[riga]

Like that?

[Tarrasch]

Exactly like that. :)

[Tarrasch]

Thanks @riga! :)