googlecompute: Correct values for on_host_maintenance

If preemptible is true then on_host_maintenance must be TERMINATE.

Also corrected order in docs.

Closes #4620

builder/googlecompute/config.go

@@ -93,15 +93,20 @@ func NewConfig(raws ...interface{}) (*Config, []string, error) {
 	if c.ImageDescription == "" {
 		c.ImageDescription = "Created by Packer"
 	}
+
+	if c.OnHostMaintenance == "MIGRATE" && c.Preemptible {
+		errs = packer.MultiErrorAppend(errs,
+			errors.New("on_host_maintenance must be TERMINATE when using preemptible instances."))
+	}
 	// Setting OnHostMaintenance Correct Defaults
-	//   "MIGRATE" : Possible if Preemptible is false
-	//   "TERMINATE": Posssible if Preemptible is true
-	if c.OnHostMaintenance == "" && c.Preemptible {
+	//   "MIGRATE" : Possible and default if Preemptible is false
+	//   "TERMINATE": Required if Preemptible is true
+	if c.Preemptible {
+		c.OnHostMaintenance = "TERMINATE"
+	} else {
+		if c.OnHostMaintenance == "" {
 			c.OnHostMaintenance = "MIGRATE"
 		}
-
-	if c.OnHostMaintenance == "" && !c.Preemptible {
-		c.OnHostMaintenance = "TERMINATE"
 	}
 
 	// Make sure user sets a valid value for on_host_maintenance option

website/source/docs/builders/googlecompute.html.md

@@ -177,15 +177,14 @@ builder.
 -   `omit_external_ip` (boolean) - If true, the instance will not have an external IP.
     `use_internal_ip` must be true if this property is true.
 
--   `preemptible` (boolean) - If true, launch a preembtible instance.
-
 -   `on_host_maintenance` (string) - Sets Host Maintenance Option. Valid
     choices are `MIGRATE` and `TERMINATE`. Please see [GCE Instance Scheduling
     Options](https://cloud.google.com/compute/docs/instances/setting-instance-scheduling-options),
-    as not all machine_types support `MIGRATE` (i.e. machines with GPUs).  The
-    default value depends on preemtability.
-    - when preemptible == true, defaults to `TERMINATE`
-    - when preemptible == false, defaults to `MIGRATE`
+    as not all machine_types support `MIGRATE` (i.e. machines with GPUs).
+    If preemptible is true this can only be `TERMINATE`. If preemptible
+    is false, it defaults to `MIGRATE`
+
+-   `preemptible` (boolean) - If true, launch a preembtible instance.
 
 -   `region` (string) - The region in which to launch the instance. Defaults to
     to the region hosting the specified `zone`.