1. Add status 'creating' for ScalingPolicy.
2. Do nothing when call ScalingGroup.Scale when ScalingGroup is disable.
3. Change the obj about GuestDetachScalingGroupTask from Guest to
ScalingGroup.
4. Fix some problem about ScalingTimer and add test for its Update func.
5. Fix problem when ScalingTimer work in controller.
1. ScalingPolicy's CoolingTime will limit ScalingPolicy with 'alarm' TriggerType
2. Health Check will remove instance that has been shut down for 3 minutes.
3. If TargetNumer > SG.MaxInstanceNumber or TargetNumber < SG.MinInstanceNumber,
ScalingActivity's status will be set to 'part_success'.
4. When you want to increase the DesireInstanceNumber that has reached the maximum value,
or decrease the this that has reached the minimum value, the ScalingActivity is rejected.
5. ScalingPolicy.PostCreate and PostDelete asynchronous execution.
