[OCP-4.x][AWS] Post-install workload nodes should be deployed to zone 'a'

[copejon]

Version: 23de699

Issue:
The AWS post-install workload machine-set template is hard coded to deploy nodes in zone ...{{aws_region.stdout}}d. Unfortunately, AWS regions differ in zone count and may not have a d zone (us-east-2 for instance).

Location:
https://github.com/openshift-scale/scale-ci-deploy/blob/23de699bb57043e6a30ebaa3b9053791f2d17a0d/OCP-4.X/roles/post-install/templates/aws-workload-node-machineset.yml.j2#L26

Result:
Post-installation phase will create the machineset and poll until it reaches the max retry count (default 600) at which point the installation process fails. The error is reported in the machineset-operator logs as a 404 response to the AWS api call.

Suggested Fix
Temporarily patch the hardcoded AZ to ...{{aws_region.stdout}}a until a more robust solution can be implemented.