Add CLOUDRON_MAIL_SMTP_STARTTLS env

starting 6.3, the internal mail server will do STARTTLS. this env
allows apps to configure themselves appropriately for pre 6.3 and
post 6.3 appropriately.

we trigger a re-configure which ensures that the new env gets put
in the database and then in the container.

src/services.js

@@ -1084,7 +1084,8 @@ function setupSendMail(app, options, callback) {
             { name: `${envPrefix}MAIL_SMTP_USERNAME`, value: app.mailboxName + '@' + app.mailboxDomain },
             { name: `${envPrefix}MAIL_SMTP_PASSWORD`, value: password },
             { name: `${envPrefix}MAIL_FROM`, value: app.mailboxName + '@' + app.mailboxDomain },
-            { name: `${envPrefix}MAIL_DOMAIN`, value: app.mailboxDomain }
+            { name: `${envPrefix}MAIL_DOMAIN`, value: app.mailboxDomain },
+            { name: `${envPrefix}MAIL_SMTP_STARTLS`, value: '1' },
         ];
         debugApp(app, 'Setting sendmail addon config to %j', env);
         appdb.setAddonConfig(app.id, 'sendmail', env, callback);