Installation fails if < 3 workers defined and number of compute replicas not set

This may be something we want to either add a validation for or document. It was initially found at a customer site but I've also confirmed it happens with just a Compact config with no workers. 

They created an agent-config.yaml with 2 worker nodes but did not set the replicas in install-config.yaml, i.e. they did not set 
compute:
- hyperthreading: Enabled
  name: worker
  replicas: {{ num_workers }} 

This resulted in an install failure as by default 3 worker replicas are created if not defined
https://github.com/openshift/installer/blob/master/pkg/types/defaults/machinepools.go#L11

See the attached console screenshot showing that the expected number of hosts doesn't match the actual.

I've also duplicated this with a compact config. We can see that the install failed as start-cluster-installation.sh is looking for 6 hosts.

[core@master-0 ~]$ sudo systemctl status start-cluster-installation.service
● start-cluster-installation.service - Service that starts cluster installation
   Loaded: loaded (/etc/systemd/system/start-cluster-installation.service; enabled; vendor preset: enabled)
   Active: activating (start) since Wed 2023-03-15 14:40:04 UTC; 3min 41s ago
 Main PID: 3365 (start-cluster-i)
    Tasks: 5 (limit: 101736)
   Memory: 1.7M
   CGroup: /system.slice/start-cluster-installation.service
           ├─3365 /bin/bash /usr/local/bin/start-cluster-installation.sh
           ├─5124 /bin/bash /usr/local/bin/start-cluster-installation.sh
           ├─5132 /bin/bash /usr/local/bin/start-cluster-installation.sh
           └─5138 diff /tmp/tmp.vIq1jH9Vf2 /etc/issue.d/90_start-install.issueMar 15 14:42:54 master-0 start-cluster-installation.sh[3365]: Waiting for hosts to become ready for cluster installation...
Mar 15 14:43:04 master-0 start-cluster-installation.sh[4746]: Hosts known and ready for cluster installation (3/6)
Mar 15 14:43:04 master-0 start-cluster-installation.sh[3365]: Waiting for hosts to become ready for cluster installation...
Mar 15 14:43:15 master-0 start-cluster-installation.sh[4980]: Hosts known and ready for cluster installation (3/6)
Mar 15 14:43:15 master-0 start-cluster-installation.sh[3365]: Waiting for hosts to become ready for cluster installation...
Mar 15 14:43:25 master-0 start-cluster-installation.sh[5026]: Hosts known and ready for cluster installation (3/6)
Mar 15 14:43:25 master-0 start-cluster-installation.sh[3365]: Waiting for hosts to become ready for cluster installation...
Mar 15 14:43:35 master-0 start-cluster-installation.sh[5079]: Hosts known and ready for cluster installation (3/6)
Mar 15 14:43:35 master-0 start-cluster-installation.sh[3365]: Waiting for hosts to become ready for cluster installation...
Mar 15 14:43:45 master-0 start-cluster-installation.sh[5124]: Hosts known and ready for cluster installation (3/6)

Since the compute section in install-config.yaml is optional we can't assume that it will be there 
https://github.com/openshift/installer/blob/master/pkg/types/installconfig.go#L126

4.12

1. Remove the compute section from install-config.yaml
2. Do an install
3. See the failure