Tectonic Terraform: aws_launch_configuration.worker_conf: Error creating launch configuration: ValidationError: The key pair does not exist

A key error message like below is usually an indicator of a mismatched region. Verify that the `AWS_REGION` env var and the `tectonic_aws_region` in the terraform.tfvars match: 

echo $AWS_REGION
eu-central-1

$ cat build/${CLUSTER}/terraform.tfvars | grep tectonic_aws_region
tectonic_aws_region = "eu-central-1"

Error applying plan:

2 error(s) occurred:

* module.workers.aws_launch_configuration.worker_conf: 1 error(s) occurred:

* aws_launch_configuration.worker_conf: Error creating launch configuration: ValidationError: The key pair 'the_key' does not exist
status code: 400, request id: 2099a467-3a64-11e7-87a0-e5189b9388c4
* module.masters.aws_launch_configuration.master_conf: 1 error(s) occurred:

* aws_launch_configuration.master_conf: Error creating launch configuration: ValidationError: The key pair 'the_key'' does not exist
status code: 400, request id: 20fa6224-3a64-11e7-8d3e-c1ca0fbfc88d

Terraform does not automatically rollback in the face of errors.
Instead, your Terraform state file has been partially updated with
any resources that successfully completed. Please address the error
above and apply again to incrementally change your infrastructure.