Make ROOT_PATH, MODELS_PATH and DOWNLOADS_PATH environment variables

Created by: Guitaricet

All config files I've seen so far have the following variables:

"ROOT_PATH": "~/.deeppavlov",
"DOWNLOADS_PATH": "{ROOT_PATH}/downloads",
"MODELS_PATH": "{ROOT_PATH}/models"

Should we make them environment variables? This would be better for the following reasons:

1. No need to define the same variables across all configs
2. No need to redefine variables for your system. For example, I do not store source code and downloads/models at same place (cause of their huge size), so I need to change configs for me and change them back to make PR (that is kind of ridiculous). If these variables were in the environment, I'd have to change them only one time - after deeppavlov installation.
3. Make configs working directory independent (no '~/'-paths)