Add support for worker pools on Google Cloud

[dghubble]

• Allow "pools" of homogeneous workers to be created using the google-cloud/kubernetes/workers Terraform (sub)module
• Examples: Add workers with a different machine type, larger disk, different preemptibility, GPUs (upcoming), etc.

Usage

Given a cluster "google-cloud-yavin" was created following the tutorial, worker pool(s) can be attached with:

module "yavin-worker-pool" {
  source = "git::https://github.com/poseidon/typhoon//google-cloud/container-linux/kubernetes/workers?ref=v1.9.4"

  # Google Cloud
  network      = "${module.google-cloud-yavin.network_name}"
  region       = "us-central1"
  count        = 2
  machine_type = "n1-standard-16"
  preemptible  = true

  cluster_name = "yavin-16x"
  ssh_authorized_key = "${var.ssh_authorized_key}"

  kubeconfig = "${module.google-cloud-yavin.kubeconfig}"
}

rel #127