Make cron management more flexible by using cron_d provider instead of templates

This patch allows for more flexibility in cron configuration when using chef-magento.

Replaces the template based cron override by using the cron_d resource provided by Opscode's cron cookbook. This way the cronjob is appended to cron.d instead of replacing the root cron file.

Compatible with Chef 10.

Note: If upgrading from older version, the previously configured crontab must be manually cleaned.