heat engine : AutoScalingGroup implement Cooldown property

AutoScalingGroup schema accepts Cooldown but it doesn't do anything
so implement Cooldown in a similar way to Scaling Policy

fixes bug 1097850

Change-Id: I0dae79fad5c13b0ae588b1a2ea4fe67eb06e3746
Signed-off-by: Steven Hardy <shardy@redhat.com>