Nextcloud easy_install.sh generates invalid cron

*Sep 15 08:30:01 turris crond[6573]: (/usr/bin/su) ERROR (getpwnam() failed - user unknown): No child process*

Nextcloud script added in 09c0408a generates invalid cron file. Why is there su and shell? Cron is able to change user and runs command in shell instance.

There are few points to that commit:

• There is no migration of original installations as far as I see so this is not applied to old users and update is not possible.
• Cron is able to change user it runs command as so that should be used instead of su
• su takes user as the first argument not as the last one

Once again I have to point out: There has to be a way to apply these changes to existing users not just the new ones!