[Resolves #937] Reset Jinja2 variable cache between config files

[NathanWilliams]

Fix for: #937

This resets the Jinja2 templating_vars in config/reader.py to prevent variables leaking across StackGroups.

PR Checklist

• Wrote a good commit message & description [see guide below].
• Commit message starts with [Resolve #issue-number].
• Added/Updated unit tests.
• Added/Updated integration tests (if applicable).
• All unit tests (make test) are passing.
• Used the same coding conventions as the rest of the project.
• The new code passes flake8 (make lint) checks.
• The PR relates to only one subject with a clear title.
  and description in grammatically correct, complete sentences.

Approver/Reviewer Checklist

• Before merge squash related commits.

Other Information

Guide to writing a good commit

[ngfgrant]

Thank you @NathanWilliams this has been a really subtle one that is catching folk out.

I think this resolves #813 and #889

I wonder if we could structure a test with this so that we can be sure it fixes the issue?

[NathanWilliams]

@ngfgrant Sure. I have been on a break, so I'll try and fit it in in the next day or so