Add config.php flag to disable some features if running as cluster

[rullzer]

I ran into this several times. But if you have a cluster of Nextcloud nodes it would be good if we could set a config.php setting for this.

This could then disable installing apps from the appstore for example (it could still check for updates just not allow you to install anything).

It might be usefull in other places as well but this is the most prominent place.