heat engine : ScalingPolicy implement Cooldown property

Implement the Cooldown property, which the schema currently
marks as implemented, but it doesn't actually do anything

ref bug 1097850

Change-Id: I0cb1b24baa4377be4a78b2a7b92fb1ed5371fb6f
Signed-off-by: Steven Hardy <shardy@redhat.com>
Merge "heat engine : ScalingPolicy implement Cooldown property"