Add support for AWS spot instance

[nicosingh]

The idea is to use aws_spot_instance_request terraform resources, the same way as we do for aws_instance resources.

For instance, this spot request:

resource "aws_spot_instance_request" "cheap_worker" {
  ami                  = "ami-1234"
  key_name             = "cheap-worker-ssh-key"
  security_groups      = ["cheap-worker-security-group"]
  wait_for_fulfillment = true
  spot_type            = "one-time"
  instance_type        = "t3.medium"

  tags {
    Role = "cheap-worker"
  }
}

Will match later in ansible-playbook using the defined Role:

---
- hosts: role_cheap-worker
  remote_user: ubuntu
  become: yes
  become_user: root
  tasks:
    - name: install apps
       apt: name=cowsay update_cache=yes

[adammck]

This is a great idea. Thank you!