feat: Configure atlas using environment variables

[OmarIthawi]

User story

• As a maintainer I need to specify which languages I'm pulling from a specific Open edX release in MFEs (and other repos) during deployment time.

Target audience

This feature targets maintainers and should help them avoid needing to fork just for the sake of updating translations.

Alternatives considered

We have two options for this feature:

• Option 1: No changes to atlas. Ask maintainers to craft their own atlas pull command, which needs no changes to atlas.
• Option 2: Let maintainers configure atlas via new environment variables which would save them few steps in case all they needed is a language change or specifying which branch to pull from.

Option 2: Environment variables

The environments needed are:

• ATLAS_FILTER: Overrides --filter
• ATLAS_REPOSITORY: Overrides --repository
• ATLAS_BRANCH: Overrides --branch