Add node autoscaling

autoscaling variable can be set when using module specifying both min and max node counts:

module "cluster" {
  source = "git::ssh://git@gitlab.developers.cam.ac.uk/uis/devops/infra/terraform/gke-cluster.git"

  project  = local.project
  location = "europe-west2"

  machine_type = "n1-standard-1"

  autoscaling = {
    min_node_count = 1
    max_node_count = 3
  }
}

Behaves as normal without having autoscaling variable set.

Also added functionality to be able to specify basic node_count (default 1) if not using autoscaling.

Closes #10 (closed)