Added support for installing updates automatically

Previously both cron-apt and yum-cron only downloaded the updates but did not
install them. Refactored this module, adding a new parameter, $install, to
control whether updates are only downloaded ($install == 'no', default) or also
installed ($install = 'yes').

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