Use random value for cron next call when value is not defined

When the schedule of a task does not define a value for delta variable, this value is not changed and so it keeps the same when the task was started. But this can give the feeling that this value is fixed as the cron should run every minutes but it can slowly drift. So it will be better to have a more random value to avoid wrong assumption. Also it will allow to add some randomness to some tasks which may be good if they communicate with external services.