Group all controlling environment variables in single file

[vnlitvinov]

Is your feature request related to a problem? Please describe.
Currently it's hard both for Modin developer and for end user to see which variables exist and what they do.
I propose to move all variables to a single unified file and replace their usage with getter functions which will have docstrings explaining what variables do.

[devin-petersohn]

This will also be important for documentation purposes as well.

[devin-petersohn]

I see a couple of options available, not sure which one is better (or maybe we can have both):

1.) Have some kind of global modin.options module where all environment variables set options, and then can be updated with set or retrieved with get. This should make it easy to change to config files as well.
2.) Context-specific configurations via with statement arguments.

A couple of questions that should influence design come to mind:

• How to ensure local configuration updates are propagated to the whole cluster? It should be easy but we have to make sure it gets done.
• Can we make certain configurations conditional on the data or workload parameters?

[vnlitvinov]

After some discussion with Devin my current implementation idea is the following:

1. Move Publisher from modin/__init__.py and generalize it enough so it can retrieve values from os.environ by itself
2. Make all env variables used now utilize this Publisher
3. Create a follow-up thread/ticket to discuss and design the (eventual) Modin configuration system, as for now it's almost non-existant (being managed by randomly placed environment variables), and gathering them in one place is just the first step - we'd like to be able to update the variable and have it propagated to all workers, have some dependencies (like make Modin pick Ray as engine if OmniSci is picked as backend), etc.