Nomad autoscaling

In order to actually make better use of our new clusters we should take advantage of autoscaling the AWS autoscaling groups. Right now they are hardcoded using the terraform `core` workspaces.

My proposed approach is to run an instance of the nomad autoscaling daemon on the monitoring server since it will have access to metrics across the cluster already. I haven't had time to look into this yet, so I'm not able to determine what kind of configuration this will require, but the core instances will most likely need additional IAM privileges for controlling the amount of instances in the groups. Those are still in the per-cluster `iam.nix` file. (which we should probably pull into bitte proper).

- [x] Update IAM permissions for core group with whatever is needed for autscaling
- [x] Move `iam.nix` to bitte repo and import from clusters.
- [ ] Ensure the new ZFS-based ASG AMIs are available in all regions (they can only be copied in the development aws org).
- [x] Update the package for `nomad-autoscaler` in this repo.
- [x] Write a NixOS module for it (probably heavily based on https://github.com/hashicorp/nomad-autoscaler/blob/master/demo/remote/aws_autoscaler.nomad and maybe running a consul-template for the config).
- [ ] Determine what metrics other than CPU/Memory we need to decide on the required number of instances.
- [ ] Bonus points for boosting the startup performance of the ASG AMIs by unifying the caching and utilizing seaweedfs.

See https://github.com/hashicorp/nomad-autoscaler