If the only thing that changed is the container count, do no make a new task def

[davidsiaw]

From #633

Before the command, task count is 1. So I expected this commands start two more tasks.
But it started three tasks and stopped the old task after new tasks started.

So, if we are running 80 tasks and increase it to 120, we need enough container instances for 200 (80+120) tasks to complete the update.

Originally posted by @essa in #633 (comment)