https://stackoverflow.com/questions/45177941/terraform-conditional-provisioning

Hi [Sparrowform](https://github.com/melezhik/sparrowform) is lightweight provisioner for Terraform based infrastructure. The benefits against other provison tools, it's decouple stage of `terraform apply` which does infrastructure bootstrap from provision stage, so you may do this:

    $ terraform apply # does infra bootstrap, it's implied no provisioner inside 
    $ sparrowform --ssh_user=my-user --ssh_private_key=/path/to/key  # do provision

Obviously you are free not to run `sparrowform` in subsequent runs. It does it's job (install ansible related dependencies, that is it). Then you drop your initial `ssh_private_key` and goes with new private key (  ansible related I guess ?)