Replace deprecated terraform resources

[timebertt]

How to categorize this issue?

/area quality
/kind enhancement
/priority normal
/platform azure

What would you like to be added:
The deprecated terraform resources network_security_group_id and route_table_id should be replaced by azurerm_subnet_network_security_group_association and azurerm_subnet_route_table_association respectively.

Why is this needed:
When creating an azure Infrastructure terraform gives out the following warnings:

Warning: "network_security_group_id": [DEPRECATED] Use the `azurerm_subnet_network_security_group_association` resource instead.

  on tf/main.tf line 24, in resource "azurerm_subnet" "workers":
  24: resource "azurerm_subnet" "workers" {



Warning: "route_table_id": [DEPRECATED] Use the `azurerm_subnet_route_table_association` resource instead.

  on tf/main.tf line 24, in resource "azurerm_subnet" "workers":
  24: resource "azurerm_subnet" "workers" {

[ialidzhikov]

They should exist both until we upgrade our terraform-provider-azurerm to v2.0.0. We already had a major issue when we removed them in the past.

[dkistner]

Yep this is already in preparation.
/assign

[ialidzhikov]

@dkistner , this can be closed after #192, right?