config/update: add "off" as a valid strategy

It was assumed that the user would specify the reboot strategy as an
unquoted value. In the case that they turn off updates, `off` is
interpreted as a boolean and the normalization pass converts that to
`false`. In the event that the user uses `"off"`, it's interpreted as a
string and not modified.

config/config_test.go

@@ -513,6 +513,7 @@ func TestNormalizeKeys(t *testing.T) {
 		{"a:\n  b:\n  - key-name: the-value\n", "a:\n  b:\n  - key_name: the-value\n"},
 
 		{"coreos:\n  update:\n    reboot-strategy: off\n", "coreos:\n  update:\n    reboot_strategy: false\n"},
+		{"coreos:\n  update:\n    reboot-strategy: 'off'\n", "coreos:\n  update:\n    reboot_strategy: \"off\"\n"},
 	} {
 		out, err := normalizeConfig(tt.in)
 		if err != nil {

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,false"`
+	RebootStrategy string `yaml:"reboot_strategy" env:"REBOOT_STRATEGY" valid:"best-effort,etcd-lock,reboot,off,false"`
 	Group          string `yaml:"group"           env:"GROUP"`
 	Server         string `yaml:"server"          env:"SERVER"`
 }