(#3010) cron: allow the absent value for the "special" property

The crontab provider fails to apply the change to a cronjob if
the schedule has been a special schedule such as @daily. Paraphrasing
from the bug report, assuming this crontab:

@reboot /bin/true

this manifest won't work correctly:

cron{ "test": minute  => 50 }

even though puppet will correctly detect that the 'minute' property
needs changing from "absent". The special schedule will remain.

The fix allows this workaround:

cron{ "test": minute  => 50, special => absent }