Add Task object for tracking task status

Task object is stored as `task' property in engine, and status can
be updated with this objects method.

Blueprint db-status-track

Change-Id: I1125d636c5b231f4dd1f2d10963b70292fea01f6
Add task status tracking in TestEngine

Blueprint db-status-track

Change-Id: Id7c97434889f56816ce80bb929e65551d5fd9072
Merge "Add Task object for tracking task status"
Merge "Add task status tracking in TestEngine"