How to use cloud-init with Terraform?
Cloud-init files are essentially bootstrap codes, that run before each startup, and can - among others - modify files, set up services, create users, etc.

Not all types of droplets support all functionalities of cloud-init, for example CoreOS uses it's own implementation, with a very limited subset of valid values.

To use this in terraform, simply provide the cloud-init file during droplet creation:

main.tf:

resource "digitalocean_droplet" "web" {
  image              = "coreos-stable"
  name               = "web"
  region             = "lon1"
  size               = "2gb"
  private_networking = true
  ssh_keys           = ["${digitalocean_ssh_key.dodemo.id}"]
  user_data          = "${file("web.conf")}"
}


web.conf:

#cloud-config
coreos:
  units:
    - name: "etcd2.service"
      command: "start"
    - name: "fleet.service"
      command: "start"


This will for example create a droplet, where CoreOS will run etcd2 and fleet during startup

You can find some more examples in this repository, where I show how one can use these configuration options to set up some simple docker based services on CoreOS