[Resolves #359] Implements use previous paramater feature

[RobReus]

Resolves #359

This new feature will make it possible to create and maintain stacks using Sceptre while also having the possibility to modify parameters outside of Sceptre, without Sceptre then overwriting/reverting these changes.

An example use-case: create and maintain your containerized environment using Sceptre, but after the initial deploy, maintain the Docker image tag using a CICD pipeline. When configuring the image tag parameter correctly, it will be given an initial value upon creation, but updates will not overwrite the value.

It is as simple as this:

parameters:
  Param:
    initial_value: "foobar"
    use_previous_value: true

It is fully backwards compatible and will not break existing functionality.

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.

[ngfgrant]

First visual pass - this looks great thanks. I’ll try get hands on with it soon.

[RobReus]

@ngfgrant any update on this? Anything that could be done by me or others to speed up the review and release process?

Happy to help where I can.