[Blueprints, 02-at-scale]: Use Secrets to pass terraform variables to Casc (part I)

[carlosrodlop]

It resolves:

• Issue [Blueprints, 02-at-scale]: Casc not working for variables at item level #70
• Issue [Blueprints, 02-at-scale]: Use Secrets to pass terraform variables to Casc #130

Now, the value of the name of the s3 bucket name can be passed to Casc. Deployment docs are shorter.