Include version stack name for "Launch Stack" button

[DanielHeath]

When upgrading from one version of the elastic stack to another, I create a new cloudformation stack, test it, then delete the old one.

The current 'Launch Stack' button always uses the stack name "buildkite".

If you do not notice the name has already been used, the AWS console will let you fill out every parameter, and then reject your stack at the last step because the name has already been used. Go back to the start and fill it out all 52 parameters again.

A simple way to avoid this would be to name the stack with the version, eg buildkite-v5-2-7, so that those who are using it to upgrade do not get name collisions.