AGW custom probes can set the host name to 127.0.0.1 by default

"Picking" the host name from the backend pool means the "Host" header
that's sent to the backend is set to the IP of the instance itself,
which messes up our reverse-proxy redirection, etc.

e.g.:

`redirect_to /login` will redirect to 'https://10.1.1.1/login' instead
of 'https://my.url.com/login'

resources.tf

@@ -73,8 +73,7 @@ resource "azurerm_application_gateway" "this" {
     timeout             = "${var.health_probe_timeout}"
     unhealthy_threshold = "${var.health_probe_threshold}"
     path                = "${var.health_probe_path}"
-
-    pick_host_name_from_backend_http_settings = true
+    host                = "127.0.0.1"
   }
 
   backend_http_settings {
@@ -85,8 +84,6 @@ resource "azurerm_application_gateway" "this" {
     request_timeout       = "${var.backend_request_timeout}"
     probe_name            = "${local.health_probe_name}"
 
-    pick_host_name_from_backend_address = true
-
     connection_draining {
       enabled           = "${var.enable_connection_draining}"
       drain_timeout_sec = "${var.connection_drain_timeout}"