Added support for displaying only changes in puppetagent::cron

This change does away with the $report_only_errors parameter and replaces it
with $report parameter that has three valid values: "everything", "changes" and
"errors" (default). Note that this change may change the behavior of existing
node that are using non-default values for $report_only_errors.

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