block public access on template bucket

[cornerman]

Sceptre creates an S3-Bucket for cloudformation templates (if template_bucket_name is set in the stack-group config). If the bucket does not exist, it will be created by sceptre automatically. This bucket is only for referencing cloudformation templates in the deployment and does not need to be public.

To assure that there is no public access, this PR configures newly created template buckets to block public access.

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.

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• Before merge squash related commits.

Other Information

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[ngfgrant]

Hi @cornerman

Going to close this see #730 for explanation. Like the idea though!

Thanks
Niall