Add support for placement groups

[jicowan]

Enhancement: allow the autoscaler to add instances to a placement group (cluster or spread) when scaling up. Could potentially implement this as an annotation that gets passed to the ASG. Placement group can be passed when then ASG is created. See PlacementGroup.

The name of the placement group into which you'll launch your instances, if any. For more information, see Placement Groups in the Amazon Elastic Compute Cloud User Guide.

Type: String