Prevent Marathon to scale down instances to zero (#62)

When :
- a deployment is stuck in an unsuccessful state (constraint
impossible to satisfy)
- maximumOverCapacity is used
- a new deployment has been requested and cancelled

The periodic scale thread removes all instances because number of
instances to decom is processed as max(goalRunning-scaleTo,
sentencedTasks).

This patch aims to change this behaviour by using number of actual
instances running instead of target number of instances (goalRunning).

JIRA: MESOS-5576