Add a function for calculating a rally point instance as part of an AWS Auto Scaling Group

[yardbirdsax]

Similar to the logic here, a function should exist that can be used to calculate a single "rally point" instance out of a pool of instances in an AWS Auto Scaling Group. This would be useful in cases where commands need to be run once as part of a bootstrapping process (e.g. something like hashicorp/terraform-aws-consul#119).