initial experiments on switchover to environment vars

[jvanasco]

this is my initial experiment on using env vars.

i'm calling this an experiment, because it works... but I'm not entirely sold on it. You can view the README formatted here: https://github.com/jvanasco/acme-dns-certbot-joohoi/tree/feature-env_vars

I changed around a few things

• everything is now driven by env vars
• i moved a chunk of code into main, as it's only used there (and was easier to do the following...)
• i added two commands that can be called by invoking the script:

$ python /etc/letsencrypt/acme-dns-auth.py --version
$ python /etc/letsencrypt/acme-dns-auth.py --setup

--version will print the version of the script's compatible env vars. I added version tracking so issues with upgrading in the future can be detected and reported

--setup will generate a template helper to setup the environment variables