Add a way to import Airflow without side-effects

[andrewgodwin]

I know it's been a long-standing issue that it should be possible to import Airflow as a library without side-effects, and while I think the ultimate fix for this is to go through and steadily remove the need to call settings.initalize() in __init__.py, I am currently working on a project where I would really like to use Airflow without it doing strange things to logging, sys.path, or atexit.

As such, this PR wraps the import side-effect in a simple environment variable check that code can use to disable the side-effects for now, while we slowly try and progress towards a "cleaner" solution. Without this, I am having to dynamically modify the source code of __init__.py in an import hook, and nobody wants that!

I don't believe it's possible to write tests for this as Airflow is already imported when tests are running, but if you can think of a way, let me know and I'll have a go. The environment variable name is also up for consideration - I just picked something that looked a bit like a setting, but we can deliberately make it not look like that if we want.

[ashb]

As tactical fixes go, sure. Probably worth adding a comment about "warning, doing this will probably have odd side effects" if someone goes spelunking.

I think it's also favour a _ prefix on the env var to make it "private"

[andrewgodwin]

I like both those ideas - I have added a warning and the private prefix.

[potiuk]

Yeah. good tactical solution :). But eventually I think we should lazy intialize everything on first use (but that's far more complex change).