Synchronize zonal and regional node pool auto_upgrade default

The regional and zonal node pool configurations had different defaults
for `auto_upgrade`. We prefer auto-upgrading node pools in general, and
moreover having different defaults for regional and zonal node pools is
very surprising.

This commit reconciles the difference by changing the zonal node pool
auto_upgrade default to true.

cluster_zonal.tf

@@ -108,7 +108,7 @@ resource "google_container_node_pool" "zonal_pools" {
 
   management {
     auto_repair  = "${lookup(var.node_pools[count.index], "auto_repair", true)}"
-    auto_upgrade = "${lookup(var.node_pools[count.index], "auto_upgrade", false)}"
+    auto_upgrade = "${lookup(var.node_pools[count.index], "auto_upgrade", true)}"
   }
 
   node_config {