Feature disable backup

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

attributes/default.rb

@@ -269,6 +269,7 @@ default['gitlab']['monitrc']['emergency_email'] = nil
default['gitlab']['monitrc']['emergency_events'] = ['timeout']
# backups
+default['gitlab']['backup']['enable'] = true
default['gitlab']['backup']['cron']['action'] = :create
default['gitlab']['backup']['cron']['minute'] = 0
default['gitlab']['backup']['cron']['hour'] = 2