Fix cloudformation ImageId regression
Fix cloudformation ImageId regression

The following commit removed the ability to use the
value set by the ImageId parameter.
https://github.com/buildkite/elastic-ci-stack-for-aws/commit/7d2ec14d1dc87ae32b98e7b75905e3d1fe6c8d51

Signed-off-by: Jeremiah Snapp <a9df78b4b5c00745f26b0821b2cc57336a474862@chef.io>

holmesjr/elastic-ci-stack-for-aws,holmesjr/elastic-ci-stack-for-aws,buildkite/elastic-ci-stack-for-aws