fix: only reference ASGBucket when needed in user data

Closes #50 (closed)

Why

When supplying a existing S3 bucket name the deployment failed with:

custom_message: AmazonCloudFormationException Template format error: Unresolved resource dependencies [ASGBucket] in the Resources block of the template (Service: AmazonCloudFormation; Status Code: 400; Error Code: ValidationError; Request ID: dd1223bd-4917-4c70-9978-2cd9ee3ef2d8; Proxy: null),

This is because the ASGBucket was referenced inside the UserData script without checking beforehand whether it should be created or not.

What

I used the variable mapping feature of Fn::Sub to only reference the ASGBucket when it is really created