Restore the ability to change InstanceType on a running stack
Restore the ability to change InstanceType on a running stack

In PR #710 we added the option to select multiple instance types by
providing the InstanceType stack param as a CSV.

Unfortunately, after merging we discovered that stack updates that
changed only the InstanceType stack param would fail with a cryptic
error:

> Internal Failure

Helpful!

After some experimentation, I realised my original PR was incorrectly
skipping the instance type overrides for types that weren't provided by
the user.

Turns out, it's possible to use YAML for 15 years and *still* get
confused about lists 😳

Fixes #774

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