Adds support for cooldown-time variable in scaling-policy

Cooldown time defines the minimum time that needs to happen between
each scaling action. If an alarm triggers before this has happened,
no action should be executed. When POL receives an alarm notification
it will check that the difference between the current time and the
last scaling action of the alarm's scaling policy is bigger than the
cooldown time.
During configuration of the scaling groups, POL will now store the
cooldown time of the scaling policy. Also, it adds a new last_scale
var to ScalingPolicy, which has as default the oldest time supported
by Python datetime.

Signed-off-by: Benjamin Diaz <bdiaz@whitestack.com>
Change-Id: I6d110ccc7f89af29500c602d05054e7ba78808a0