Devise automation for instance refresh

[xmunoz]

Currently, when a new Terraform deployment is triggered, the instances in the autoscaling groups will not refresh without clicking "Start Instance Refresh" in the AWS Autoscaler UI.

There is an open issue for this, but Terraform currently does not provide the functionality yet to do this automatically.

Figure out a way to automate instance refresh, or at least keep an eye on the linked issue to know when this feature becomes available.

[cecilia-donnelly]

This refers to new image builds, in contrast to #23.