Adds env var to set certbot acme server

this is required for test suite to use dns certbot request
without talking to live or staging letsencrypt servers or
production level dns providers. This is a backwards port
from the v3 branch and opens the door for a full certificate
cypress test

backend/lib/config.js

@@ -180,5 +180,15 @@ module.exports = {
 	 */
 	useLetsencryptStaging: function () {
 		return !!process.env.LE_STAGING;
+	},
+
+	/**
+	 * @returns {string|null}
+	 */
+	useLetsencryptServer: function () {
+		if (process.env.LE_SERVER) {
+			return process.env.LE_SERVER;
+		}
+		return null;
 	}
 };