standardised CHANGELOG exists, and is validated via pipeline

[doodlesbykumbi]

If the repo has a changelog that doesn't meet the standard, do try to change earlier entries to match the standard. If the repo doesn't have a changelog use this as a starter:

# Changelog
All notable changes to this project will be documented in this file.

The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).

## [Unreleased]

Acceptance criteria

• [x] CHANGELOG.md exists and follows the KeepAChangeLog format. For reference consult https://github.com/cyberark/conjur/blob/master/CHANGELOG.md.
• [x] Add parse-changelog validation step to Jenkinsfile. For reference consult https://github.com/cyberark/conjur/blob/master/Jenkinsfile#L27.

[JakeQuilty]

https://github.com/cyberark/summon-aws-secrets/pull/22