UpdateStack for AWS::AutoScaling::ScalingPolicy

Implemented update support of the ScalingPolicy resource
in an existing stack. ScalingAdjustment, AdjustmentType,
and Cooldown properties are allowed properties for update.
A handle_update method is added to update differences in
the template.

Change-Id: Ieeae93560a315a8a35c6fb2d8f1d78a89123bec3
blueprint: scalingpolicy-update-stack
Merge "UpdateStack for AWS::AutoScaling::ScalingPolicy"