exec/files: sort unit preset in alphabetical order

The 20-ignition.preset file is used in CoreOS layering. The contents
of this file will be sorted in alphabetical order by unit name to ensure
the file is written in a consistent order across multiple runs.

Fixes coreos#1339

internal/exec/stages/files/units.go

@@ -18,6 +18,7 @@ import (
 	"context"
 	"fmt"
 	"path/filepath"
+	"sort"
 	"strings"
 
 	"github.com/coreos/ignition/v2/config/shared/errors"
@@ -151,7 +152,16 @@ func (s *stage) createSystemdPresetFile(presets map[string]*Preset) error {
 		return err
 	}
 	hasInstanceUnit := false
-	for _, value := range presets {
+
+	// sort the units before writing to the systemd presets file
+	unitNames := make([]string, 0, len(presets))
+	for unit := range presets {
+		unitNames = append(unitNames, unit)
+	}
+	sort.Strings(unitNames)
+
+	for i := range unitNames {
+		value := presets[unitNames[i]]
 		unitString := value.unit
 		if value.instantiatable {
 			hasInstanceUnit = true