feat: Upgrade to terraform 0.15 (#3)

* Upgrade provider plugins to latest release
* Improve README instructions
* Convert locals in main.tf to vars
* Add output for db proxy public IP to make connecting easier
* Add plan and crash.log to .gitignore
* Move backend config to backend.tf
* Move provider config to providers.tf
* Move required_versions to versions.tf

modules/dbproxy/outputs.tf

@@ -0,0 +1,4 @@
+output "public_ip" {
+  description = "The public IP of the bastion instance running Cloud SQL Proxy"
+  value       = google_compute_instance.db_proxy.network_interface.0.access_config.0.nat_ip
+}