vary dkim selector per mail domain

this is required for the case where the domain is added on multiple
cloudrons. initially, the plan was to just vary this as a derivation
of the dashboard domain. but this will break existing installation (wildcard
and manual domain setups cannot be re-programmed automatically).

src/constants.js

@@ -17,7 +17,6 @@ exports = module.exports = {
     ],
 
     ADMIN_LOCATION: 'my',
-    DKIM_SELECTOR: 'cloudron',
 
     NGINX_DEFAULT_CONFIG_FILE_NAME: 'default.conf',