[QT-601] Nest scenario step variables in eval context

[ryancragun]

Previously we'd expose any know scenario step variables at the top-level
of a steps entry in the eval context. This leads to problems if a
Terraform module has both an input and output with the same name.
Consider the following scenario:

module "cluster" {
  source = "./modules/cluster"
}

module "worker" {
  source = "./modules/worker"
}

variable "addr" {
  type = string
  default = "http://192.168.0.1/"
}

scenario "boundary" {
  step "cluster" {
    module = module.cluster
    variables {
      addr = var.addr
    }
  }

  step "worker" {
    module = module.worker
    variables {
      upstream_addr = step.cluster.addr
    }
  }

  step "worker_downstream" {
    module = module.worker
    variables {
      upstream_addr = step.worker.upstream_addr
    }
  }
}

We expect expect step.cluster.addr to be the known value of of
var.addr. But what happens with step worker here is the problem.
step.worker.upstream_addr should be module call to step.cluster.addr
but because we've exposed step.cluster.addr in the eval context we
don't do a module call, and instead inherit the value from var.addr.
The same happens with step.worker_downstream.upstream_addr. Instead of
being a module call to step.worker.upstream_addr we inherit the know
value of var.addr.

With this change that behavior is now default. If you wish to use the
prior behavior (getting a steps known variables) you can access them
with the variables key, e.g. step.cluster.variables.addr.

NOTE: this is technically a breaking change in behavior. We'll want to test all of Vault
before we release this. If we do run into issues we can modify scenarios to use the
variables option if they want old behavior.

• The commit message includes an explanation of the changes
• Manual validation of the changes have been performed (if possible)
• New or modified code has requisite test coverage (if possible)
• I have performed a self-review of the changes
• I have made necessary changes and/or pull requests for documentation
• I have written useful comments in the code

[moduli]

Oh neat. I didn't know you could do this. To clarify, this is just taking the value that you're inputting into step infra_default? I have been doing something like...

  locals {
    network_cluster  = "e2e_cluster"
  }

  step "create_docker_network_cluster" {
    module = module.docker_network
    variables {
      network_name = local.network_cluster
    }
  }

  step "create_boundary" {
    module = module.docker_boundary
    depends_on = [
      step.create_docker_network_cluster,
    ]
    variables {
      network_name     = [local.network_cluster]
    }
  }

but I could change that to...

  step "create_docker_network_cluster" {
    module = module.docker_network
    variables {
      network_name = "e2e_cluster"
    }
  }

  step "create_boundary" {
    module = module.docker_boundary
    depends_on = [
      step.create_docker_network_cluster,
    ]
    variables {
      network_name     = [step.create_docker_network_cluster.variables.network_name]
    }
  }

Is that correct?

[ryancragun]

you can after this ships!