====== Reminder Mail Placeholders (txt/html) ======
An example:
{$customer} stepping stone GmbH {$system} kvm-0003: stepping stone GmbH: Gentoo (stoney backup Server) {$salutation} Mr. {$surname} Eichenberger {$maintenance_date} 20160817 {$maintenance_start} 16:00 {$maintenance_end} 17:00
Optional Values: