Description of problem:

There are two ways to provide amiID while installing a cluster into GovCloud or C2S region
option 1: `controlPlane.platform.aws.amiID` and `compute[0].platform.aws.amiID`
option 2: `platform.aws.amiID`

installer reports a FATAL error while using option 1:
FATAL failed to fetch Kubeconfig Admin Client: failed to load asset "Install Config": platform.aws.amiID: Required value: AMI must be provided

This should be a regression issue, since it works well on 4.6.24 GA and 4.7.6 GA

Version-Release number of selected component (if applicable):
OCP 4.8.0-0.nightly-2021-04-09-222447

How reproducible:
Always.

Steps to Reproduce:
1. Create install-conifg.yaml

apiVersion: v1
baseDomain: qe.devcluster.openshift.com
compute:
- architecture: amd64
  hyperthreading: Enabled
  name: worker
  platform:
    aws:
      amiID: ami-0b12252a3e0823aab
  replicas: 3
controlPlane:
  architecture: amd64
  hyperthreading: Enabled
  name: master
  platform:
    aws:
      amiID: ami-0b12252a3e0823aab
  replicas: 3
metadata:
  creationTimestamp: null
  name: yunjiang-3424-mw
networking:
  clusterNetwork:
  - cidr: 10.128.0.0/14
    hostPrefix: 23
  machineNetwork:
  - cidr: 10.0.0.0/16
  networkType: OpenShiftSDN
  serviceNetwork:
  - 172.30.0.0/16
platform:
  aws:
    region: us-gov-west-1
publish: Internal
pullSecret: <HIDDEN>
sshKey: <HIDDEN>

2. Create ignition-configs

Actual results:
FATAL failed to fetch Kubeconfig Admin Client: failed to load asset "Install Config": platform.aws.amiID: Required value: AMI must be provided


Expected results:
INFO Consuming Install Config from target directory
INFO Credentials loaded from the "default" profile in file "/home/ec2-user/.aws/credentials"
INFO Ignition-Configs created in: /tmp/aaaaa3702-p and /tmp/aaaaa3702-p/auth

Additional info: