installer: concurrent parent task must wait for all children

Change-Id: I1515d36cc731c6063359dd6957743d853b0255f6
diff --git a/core/installer/tasks/tasks.go b/core/installer/tasks/tasks.go
index c097af3..1cc053b 100644
--- a/core/installer/tasks/tasks.go
+++ b/core/installer/tasks/tasks.go
@@ -157,12 +157,16 @@
 			})
 			go subtasks[i].Start()
 		}
-		return <-errCh
+		cnt := 0
 		for _ = range subtasks {
 			err := <-errCh
 			if err != nil {
 				return err
 			}
+			cnt++
+			if cnt == len(subtasks) {
+				break
+			}
 		}
 		return nil
 	}