Refactor notifier cron job creation

This commit moves the notifier cron job creation more into hiera,
because we want to add a second job that runs once a week to send
summary emails.

This commit should not change the behaviour of the existing cronjob.
Add notifier cron job for summary emails

Sends emails early on a Monday morning.
Merge pull request #666 from alphagov/notifier-cron-job-refactor

Notifier cron job refactor