roles/nginx: Add pre and post hooks for Let's Encrypt on Ubuntu 20.04

Certbot will run any executables in the pre and post directories
during certificate renewal.

roles/nginx/files/start-nginx.sh

@@ -0,0 +1,3 @@
+#!/usr/bin/env bash
+
+/bin/systemctl start nginx

roles/nginx/files/stop-nginx.sh

@@ -0,0 +1,3 @@
+#!/usr/bin/env bash
+
+/bin/systemctl stop nginx