Add a Capacity Rebalance Flag for Spot Node Allocation Strategy

[cryan1988]

Is this a BUG REPORT or FEATURE REQUEST?:
FEATURE REQUEST

What happened:
Capacity rebalance allocation strategy is currently disabled by default in the underlying ASG that is created by the instance-manager.

What you expected to happen:
This is the expected behaviour but it would be useful to have a way of setting "CapacityRebalance": true as described here:
https://docs.aws.amazon.com/autoscaling/ec2/userguide/ec2-auto-scaling-capacity-rebalancing.html

How to reproduce it (as minimally and precisely as possible):
I noticed this when creating via the EKS provisioner and going through https://github.com/keikoproj/instance-manager/blob/master/docs/EKS.md I don't see this as an option

Is there any way to set the Capacity rebalance to true on creation of the instance group? Similar to "strategy" in https://github.com/keikoproj/instance-manager/blob/master/docs/EKS.md#mixedinstancespolicyspec