Feature disable backup

This MR permits the user to disable backup cron task even if environment is production.

Probably conflicting with !109 (merged)

@matthieu.o.vachon Disabling backups is never a good idea but I understand that people like living on the edge and also have their own solutions so I like this. Can you rebase please?

Our problem is that repositories total size is 30G, doing a backup in a cron tab is too heavy from our perspective. Moreover, as you said, we already backup whole machine on a daily basis. Enabling cron backup would simply double the size of the backup on tape.

I'm glad you accepted to merge this however :) I'm going to rebase this.

Rebased

Reassigned to @marin

Milestone changed to 7.3

Thanks!