Description of the problem:
For ocp versions < 4.19 The ACM host-inventory os release field cannot be changed. The default is 4.19. 4.19 introduced the node-image-pull.service and node-image-overlay service which does not exist prior to 4.19. This causes the cluster to fail with messages:

Failure messages:systemctl start --no-block node-image-pull.service node-image-overlay.service],
Error exit status 5, LastOutput
Failed to start node-image-pull.service: Unit node-image-pull.service not found.
Failed to start node-image-overlay.service: Unit node-image-overlay.service not found.

 How reproducible:
every time for ocp 4.16 and 4.18
 

Steps to reproduce:

1. ACM standalone cluster install 4.16-4.18

2. unable to change os version when creating host inventory

3. Install fails with error
Failed to start node-image-pull.service: Unit node-image-pull.service not found.
Failed to start node-image-overlay.service: Unit node-image-overlay.service not found.

Actual results:
nstall fails with error
Failed to start node-image-pull.service: Unit node-image-pull.service not found.
Failed to start node-image-overlay.service: Unit node-image-overlay.service not found.
 

Expected results:
install succeeds as it does with 4.19