feat(pkg/daemon): leave backup copy of encapsulated MC

Rename rather than remove the encapsulated MachineConfig file after processing
on firstboot. This allows users to check what data was processed on the node
after the mcd firstboot service has run successfully.

pkg/daemon/constants/constants.go

@@ -43,6 +43,11 @@ const (
 	// non-Ignition fields such as the osImageURL and kernelArguments.
 	MachineConfigEncapsulatedPath = "/etc/ignition-machine-config-encapsulated.json"
 
+	// MachineConfigEncapsulatedBakPath defines the path where the machineconfigdaemom-firstboot.service
+	// will leave a copy of the encapsulated MachineConfig in MachineConfigEncapsulatedPath after
+	// processing for debugging and auditing purposes.
+	MachineConfigEncapsulatedBakPath = "/etc/ignition-machine-config-encapsulated.json.bak"
+
 	// MachineConfigDaemonForceFile if present causes the MCD to skip checking the validity of the
 	// "currentConfig" state.  Create this file (empty contents is fine) if you wish the MCD
 	// to proceed and attempt to "reconcile" to the new "desiredConfig" state regardless.

pkg/daemon/daemon.go

@@ -483,8 +483,8 @@ func (dn *Daemon) RunFirstbootCompleteMachineconfig() error {
 	}
 
 	// Removing this file signals completion of the initial MC processing.
-	if err := os.Remove(constants.MachineConfigEncapsulatedPath); err != nil {
-		return errors.Wrapf(err, "failed to remove %s", constants.MachineConfigEncapsulatedPath)
+	if err := os.Rename(constants.MachineConfigEncapsulatedPath, constants.MachineConfigEncapsulatedBakPath); err != nil {
+		return errors.Wrap(err, "failed to rename encapsulated MachineConfig after processing on firstboot")
 	}
 
 	dn.skipReboot = false