Fixed an annoying spamming issue in cron.pp

Previously cron would send anything printed to stderr during a puppet run would
get sent to the admin. Change the cron command to redirect strerr to stdout
first and the grep, so that only real errors (i.e. lines starting with "err")
would trigger an email.

Signed-off-by: Samuli Seppänen <samuli@openvpn.net>