[Resolve #359] Support use-previous-parameters

[alex-harvey-z3q]

I am adopting this stale PR by @RobReus
#914

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 (poetry run tox) are passing.
• Used the same coding conventions as the rest of the project.
• The new code passes pre-commit validations (poetry run 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

• Before merge squash related commits.

Other Information

Guide to writing a good commit

[alex-harvey-z3q]

@RobReus Can you recall the extent to which you tested this feature yourself, besides the unit tests you have added?

[alex-harvey-z3q]

Some of this logic around validating the parameters would need to be moved to _ensure_parameters inside the Stack class.

[alex-harvey-z3q]

To avoid potential rework here, this PR would need to go in first. #1471

[alex-harvey-z3q]

I have changed my mind about merging this considering that it adds some complexity in the code we have no known users who would benefit from the feature.