Re-add sleep of 2 sec before looking for partitions (#172)

This fixes issue #162 and #165. I refactored the timeout handling,
with #169 by increasing the existing timeout and removing the additional 2 sec
sleep, because the 2 sec sleep was not working in all cases.
Unfortunately, the increase alone is not sufficient, so I'm re-adding
the 2 sec sleep again.

pkg/builder/step_map_image.go

@@ -48,6 +48,7 @@ func (s *stepMapImage) Run(_ context.Context, state multistep.StateBag) multiste
 	loop := strings.Split(path, "/")[2]
 	prefix := loop + "p"
 
+	time.Sleep(2 * time.Second)
 	// Look for all partitions of created loopback
 	var partitions []string
 	cPartitions := make(chan []string)