Add missing provider information due to upgrade in TF version.

ohpc-instance/main.tf

 # is created in external-network module and called in root module
+terraform {
+required_version = ">= 0.14.0"
+  required_providers {
+    openstack = {
+      source  = "terraform-provider-openstack/openstack"
+      version = "~> 1.42.0"
+    }
+  }
+}
+
 variable "external_subnet_id" {type = string}
 
 variable "ohpc_instance_name" {}