Issue #1493098 by sun, jcisio, pcambra, Pol, wamilton, Rok Žlender, beejeebus: convert cron settings to configuration system.

PR for https://github.com/backdrop/backdrop-issues/issues/170
According to sun on the drupal forums: 

>Apparently, the Drupal installer does not install system module in the regular/proper procedure. Due to that, the
>system.cron:key is never actually set/updated after the Drupal installation. Attached patch additionally fixes the
>Drupal installer to install System module correctly.