chore(examples): add version constraints to ubuntu example (#707)

* Create version constraint on Random, and also move to 'required_providers' (post-Terraform 0.13 change).

* Formatting.

Author: boldandbusted

examples/ubuntu/providers.tf

@@ -1,4 +1,18 @@
+terraform {
+  required_providers {
+    aws = {
+      version = "3.20"
+    }
+    random = {
+      version = "3.1.0"
+    }
+  }
+}
+
+
 provider "aws" {
-  region  = local.aws_region
-  version = "3.20"
+  region = local.aws_region
+}
+
+provider "random" {
 }