Enum values are interpreted as strings in the web ui, which makes
impossible to chosse the number of backups.

Also, set the minimum number of backup to 0, as it is now possible.

Revert "resilient: new paramater to define the time to sleep before running the backup in the cron job."

This reverts commit 30d1a937.

Default is a random value between 0 and 6 hours. KVM and Slaprunner test instances set it at 0.