Update Control Plane replica validation for Single Node OpenShift

Installer can be used to provision Single Node OpenShift(SNO) only
on AWS, GCP, Azure, PowerVS and Baremetal platforms.
SNO is not supported on all on-prem platforms yet. Even on the
platforms that support SNO installs, the install method currently
supported are assisted/agent/ZTP.

Author: sadasu

pkg/asset/machines/master.go

@@ -185,6 +185,17 @@ func (m *Master) Generate(ctx context.Context, dependencies asset.Parents) error
 	var ipClaims []ipamv1.IPAddressClaim
 	var ipAddrs []ipamv1.IPAddress
 	var controlPlaneMachineSet *machinev1.ControlPlaneMachineSet
+
+	// Check if SNO topology is supported on this platform
+	if pool.Replicas != nil && *pool.Replicas == 1 {
+		bootstrapInPlace := false
+		if ic.BootstrapInPlace != nil {
+			bootstrapInPlace = true
+		}
+		if !supportedSingleNodePlatform(bootstrapInPlace, ic.Platform.Name()) {
+			return fmt.Errorf("this install method does not support Single Node installation on platform %s", ic.Platform.Name())
+		}
+	}
 	switch ic.Platform.Name() {
 	case awstypes.Name:
 		subnets, err := aws.MachineSubnetsByZones(ctx, installConfig, awstypes.ClusterNodeSubnetRole)
@@ -967,6 +978,20 @@ func IsFencingCredentialsFile(filepath string) (bool, error) {
 	if err != nil {
 		return false, err
 	}
-
 	return match, nil
 }
+
+// supportedSingleNodePlatform indicates if the IPI Installer can be used to install SNO on
+// a platform.
+func supportedSingleNodePlatform(bootstrapInPlace bool, platformName string) bool {
+	switch platformName {
+	case awstypes.Name, gcptypes.Name, azuretypes.Name, powervstypes.Name:
+		// Single node OpenShift installations supported without `bootstrapInPlace`
+		return true
+	case nonetypes.Name, externaltypes.Name:
+		// Single node OpenShift installations supported with `bootstrapInPlace`
+		return bootstrapInPlace
+	default:
+		return false
+	}
+}