As using aks-engine just to have extra storage was a bit of wasting resources for me, I found finally a way to add extra storage for each aks node, entirely by terraform way:

resource "azurerm_subnet" "subnet" {
  name                = "aks-subnet-${var.name}"
  resource_group_name = "${azurerm_resource_group.rg.name}"
  # Uncomment this line if you use terraform < 0.12
  network_security_group_id = "${azurerm_network_security_group.sg.id}"
  address_prefix            = "10.1.0.0/24"
  virtual_network_name      = "${azurerm_virtual_network.network.name}"
}

resource "azurerm_kubernetes_cluster" "cluster" {
  ...
  agent_pool_profile {
    ...
    count           = "${var.kubernetes_agent_count}"
    vnet_subnet_id  = "${azurerm_subnet.subnet.id}"
  }
}

# Find all agent node ids by extracting it's name from subnet assigned to cluster:
data "azurerm_virtual_machine" "aks-node" {
  # This resource represent each node created for aks cluster.
  count = "${var.kubernetes_agent_count}"
  name  = distinct([for x in azurerm_subnet.subnet.ip_configurations : replace(element(split("/", x), 8), "/nic-/", "")])[count.index]

  resource_group_name = azurerm_kubernetes_cluster.cluster.node_resource_group
  depends_on = [
    azurerm_kubernetes_cluster.cluster
  ]
}

# Create disk resource in size of aks nodes:
resource "azurerm_managed_disk" "aks-extra-disk" {
  count                = "${var.kubernetes_agent_count}"
  name                 = "${azurerm_kubernetes_cluster.cluster.name}-disk-${count.index}"
  location             = "${azurerm_kubernetes_cluster.cluster.location}"
  resource_group_name  = "${azurerm_kubernetes_cluster.cluster.resource_group_name}"
  storage_account_type = "Standard_LRS"
  create_option        = "Empty"
  disk_size_gb         = 10
}

# Attach our disks to each agents:
resource "azurerm_virtual_machine_data_disk_attachment" "aks-disk-attachment" {
  count              = "${var.kubernetes_agent_count}"
  managed_disk_id    = "${azurerm_managed_disk.aks-extra-disk[count.index].id}"
  virtual_machine_id = "${data.azurerm_virtual_machine.aks-node[count.index].id}"
  lun                = "10"
  caching            = "ReadWrite"
}