ESC: Stack refs exercise is too complicated

[jkodroff]

People seemed like they were tuning out some during the stack refs exercise. Some ways we can probably improve:

• Do an EC2 instance stack ref example because we do this earlier in the training and an EKS cluster takes forever to spin up.
• Share an EKS cluster (pre-baked) and let people consume the Kubeconfig. (Requires everyone being in the same Pulumi Org.)