Add Hetzner "addons" for the default cluster

[wenzel-felix]

Add a bool to disable default addons - normally the module should install similar components like:

resource "rancher2_cluster" "test_cluster" {
  name        = "test-cluster"
  description = "Foo rancher2 custom cluster"
  rke_config {
    addons = <<EOF
---
apiVersion: v1
stringData:
  token: ${var.hetzner_token}
  network: ${hcloud_network.main.name}
kind: Secret
metadata:
  name: hcloud
  namespace: kube-system
    EOF
    addons_include = [ "https://github.com/hetznercloud/hcloud-cloud-controller-manager/releases/latest/download/ccm-networks.yaml" ]
    services {
      kubelet {
        extra_args = {
          "cloud-provider" = "external"
        }
      }
    }
    enable_cri_dockerd = true
    network {
      plugin = "canal"
    }
  }
}