Allow configuration of 'placementStrategy' for Layer0

[diemonster]

We currently default to 'binpack' for all task placements: http://docs.aws.amazon.com/AmazonECS/latest/developerguide/task-placement-strategies.html

However, there is value in allowing the user to configure their own placementStrategy. Some discussion as to why: https://github.com/quintilesims/layer0/pull/201

This would be a major feature, as we'd have to majorly rethink how we dealt with multi-az deployments (for example) if we were going to take full advantage of the placementStrategies available.