Add mesos-slave to Amazon auto scaling group

[tayzlor]

We should be able to just provision a single slave instance, then use amazon auto-scaling to handle provisioning new slave instances dynamically on demand.

See http://docs.aws.amazon.com/AutoScaling/latest/DeveloperGuide/as-scale-based-on-demand.html and http://docs.aws.amazon.com/AutoScaling/latest/DeveloperGuide/GettingStartedTutorial.html

[wallies]

maybe need to expand on something like this https://github.com/awslabs/ecs-mesos-scheduler-driver