✨ Add installation of ES if the base raw image is missing

[beranm14]

Related: #9

[beranm14]

Tested with the following example/main.tf:

resource "google_compute_address" "outgoing_traffic_europe_west3" {
  name    = "nat-external-address-europe-west3"
  region  = var.region
  project = var.project
}

module "cloud-nat" {
  source        = "terraform-google-modules/cloud-nat/google"
  version       = "~> 1.2"
  project_id    = var.project
  region        = var.region
  create_router = true
  network       = "default"
  router        = "nat-router"
  nat_ips       = [google_compute_address.outgoing_traffic_europe_west3.self_link]
}

module "elasticsearch" {
  source  = "./.."
  project = var.project
  region  = var.region
  zone    = var.zone

  instance_name  = "elasticsearch"
  cluster_name   = "elasticsearch"
  node_count     = 2
  heap_size      = "1500m"
  data_disk_size = "10"

  namespace        = var.namespace

  raw_image_source = ""

  cluster_ca_certificate = module.gke.cluster_ca_certificate
  cluster_user           = module.gke.cluster_username
  cluster_password       = module.gke.cluster_password
  cluster_endpoint       = module.gke.endpoint

  allowed_ipv4_subnets = [module.gke.cluster_ipv4_cidr]
}