Added configurable randomized delay to puppet runs in cron subclass

Previously puppet runs were launched at the exact same time (provided ntp was
installed). This seems to cause the puppetmaster to drop client connections even
with a fairly small amount of clients. Added randomized delay to puppet runs -
by default 60 (seconds) - using the new $maxdelay parameter.

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