reboot-strategy: remove the 'false' value

Since we no longer do a two-stage unmarshal, the 'false' value will no
longer be necessary.

config/update.go

@@ -17,7 +17,7 @@
 package config
 
 type Update struct {
-	RebootStrategy string `yaml:"reboot_strategy" env:"REBOOT_STRATEGY" valid:"best-effort,etcd-lock,reboot,off,false"`
+	RebootStrategy string `yaml:"reboot_strategy" env:"REBOOT_STRATEGY" valid:"best-effort,etcd-lock,reboot,off"`
 	Group          string `yaml:"group"           env:"GROUP"`
 	Server         string `yaml:"server"          env:"SERVER"`
 }

system/update.go

@@ -126,7 +126,7 @@ func (uc Update) Units() []Unit {
 			Runtime: true,
 		}}
 
-		if uc.Update.RebootStrategy == "false" || uc.Update.RebootStrategy == "off" {
+		if uc.Update.RebootStrategy == "off" {
 			ls.Command = "stop"
 			ls.Mask = true
 		}

system/update_test.go

@@ -71,15 +71,6 @@ func TestUpdateUnits(t *testing.T) {
 				Runtime: true,
 			}}},
 		},
-		{
-			config: config.Update{RebootStrategy: "false"},
-			units: []Unit{{config.Unit{
-				Name:    "locksmithd.service",
-				Command: "stop",
-				Runtime: true,
-				Mask:    true,
-			}}},
-		},
 		{
 			config: config.Update{RebootStrategy: "off"},
 			units: []Unit{{config.Unit{
@@ -109,7 +100,7 @@ func TestUpdateFile(t *testing.T) {
 		},
 		{
 			config: config.Update{RebootStrategy: "wizzlewazzle"},
-			err:    &config.ErrorValid{Value: "wizzlewazzle", Field: "RebootStrategy", Valid: []string{"best-effort", "etcd-lock", "reboot", "off", "false"}},
+			err:    &config.ErrorValid{Value: "wizzlewazzle", Field: "RebootStrategy", Valid: []string{"best-effort", "etcd-lock", "reboot", "off"}},
 		},
 		{
 			config: config.Update{Group: "master", Server: "http://foo.com"},
@@ -143,14 +134,6 @@ func TestUpdateFile(t *testing.T) {
 				RawFilePermissions: "0644",
 			}},
 		},
-		{
-			config: config.Update{RebootStrategy: "false"},
-			file: &File{config.File{
-				Content:            "REBOOT_STRATEGY=false\n",
-				Path:               "etc/coreos/update.conf",
-				RawFilePermissions: "0644",
-			}},
-		},
 		{
 			config: config.Update{RebootStrategy: "off"},
 			file: &File{config.File{