Even i've not successed to setup running cluster right now it looks like there is only predefined node configuration roles. Master, etcd and workers have their exclusive nodes.
Here example config, that also points to that.

  tectonic_aws_etcd_ec2_type = "t2.medium"
  tectonic_aws_worker_ec2_type = "t2.medium"
  tectonic_aws_master_ec2_type = "t2.medium"

  tectonic_etcd_count = "0"
  tectonic_master_count = "1"
  tectonic_worker_count = "2"

I can understand that this is maybe preferable setup for larger clusters, but for smaller teams it might be to much and maybe, one can save nodes and cost, by assigning master, etcd, and workers to same nodes.
Do you tend to disagree probably and there are no plans to allow such kind of setup? Or would int be possible to include more flexible cluster config?