Major updates to packetfilter module

- New puppetlabs firewall modules have built-in rule persistance based on the
  'iptables-persistent' service script. Removed the rclocal-based persistence.
- Added packetfilter::absent class to remove obsolete configurations - in this
  case the /etc/rclocal/packetfilter script we don't want to run anymore.
- Made the main class ('packetfilter') safely includeable. It only loads the
  run stages, loads packetfilter::absent and ensures the 'iptables-persistent'
  package is installed.

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