:hammer: enable chart-diff to connect to production and clean up envs

[Marigold]

Motivation

Managing various envs (live, staging, local, etc.) has become more scattered and everyone is reinventing the wheel with their own helper functions. This PR unifies env management under existing OWIDEnv class.

Implementation

Leverage OWIDEnv object for managing settings and engines for various environments. Instead of using default etl.config, you can now pass arbitrary config. There are also helper functions for creating OWIDEnv for staging servers etc.

I also removed staging from OWIDEnvType since we no longer use them (@lucasrodes can I remove remote-staging to staging?). Should I maybe also remove mentions of live from everywhere and replace them by production?

Production env

Currently, we compare chart-diff against staging-site-master on staging servers. That's not optimal as we'd like to compare it against production. Hence, we add ENV_FILE_PROD env variable that points to production .env file (e.g. .env.prod.read).

TODO after merging

• [ ] Add ENV_FILE_PROD=.env.prod.read variable and actual .env.prod.read file to all staging servers