use openstack volume_size option on creation

if present. Otherwise fallback to to source image min disk value (if
set) or calculated from the source image bytes size.

Note that in some cases this needs to be specified, if
`use_blockstorage_volume` is true. See #6957 for the discussion.

builder/openstack/step_create_volume.go

@@ -35,20 +35,25 @@ func (s *StepCreateVolume) Run(_ context.Context, state multistep.StateBag) mult
 		state.Put("error", err)
 		return multistep.ActionHalt
 	}
-	imageClient, err := config.imageV2Client()
-	if err != nil {
-		err = fmt.Errorf("Error initializing image client: %s", err)
-		state.Put("error", err)
-		return multistep.ActionHalt
-	}
+
+	volumeSize := config.VolumeSize
 
 	// Get needed volume size from the source image.
-	volumeSize, err := GetVolumeSize(imageClient, sourceImage)
-	if err != nil {
-		err := fmt.Errorf("Error creating volume: %s", err)
-		state.Put("error", err)
-		ui.Error(err.Error())
-		return multistep.ActionHalt
+	if volumeSize == 0 {
+		imageClient, err := config.imageV2Client()
+		if err != nil {
+			err = fmt.Errorf("Error initializing image client: %s", err)
+			state.Put("error", err)
+			return multistep.ActionHalt
+		}
+
+		volumeSize, err = GetVolumeSize(imageClient, sourceImage)
+		if err != nil {
+			err := fmt.Errorf("Error creating volume: %s", err)
+			state.Put("error", err)
+			ui.Error(err.Error())
+			return multistep.ActionHalt
+		}
 	}
 
 	ui.Say("Creating volume...")