Add iptables rule to log dropped packets

Adds a rule to the bottom of the openstack-INPUT chain to log any
packets that make it this far and are on their way to being rejected.
This may help us find out why Devstack Ironic VM nodes occasionally cannot
reach the host's TFTP server.

Change-Id: I3980284a93f50ac8db16769f6ad87382aa55e898
Related-bug: #1393099
Merge "Add iptables rule to log dropped packets"