merge stack_group_config to sceptre_user_data if sceptre_user_data told to

[jerry153fish]

This pr is for issues like #604 and #583, sometimes we need to access properties in stack_conifg eg: region or profile or need to add to project scope shared configs

1. If sceptre_user_data config the merge_config as True, ensure sceptre_user_data access the stack_config and overwrite the stack_config properites
2. if no merge_config in sceptre_user_data or as False, ensure on regression

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).
• [x ] All unit tests (make test) are passing.
• Used the same coding conventions as the rest of the project.
• The new code passes pre-commit validations (pre-commit run --all-files).
• The PR relates to only one subject with a clear title.
  and description in grammatically correct, complete sentences.

Approver/Reviewer Checklist

• [x ] Before merge squash related commits.

[zaro0508]

I've commented a possible workaround in issue #604

I would like to note that sceptre has --var-file option which could provide a way to pass in sceptre configs to templates.

I would also like to note that there is an alternative approach for this in PR #928 which I think is better than this approach. Maybe you can help us review that one @jerry153fish ?

[jerry153fish]

@zaro0508 Agreed. The #928 is a better way than this.