Environment-specific variables

What does this MR do?

Implement environment-specific variables

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug

TODO:

• Update doc to be consistent with protected_branches/tags
• Decide if we should go with scope or say, environment_name
• Backport to CE

What are the relevant issue numbers?

Closes #2302 (closed)