Make iptables drop action configurable.

Resolves bug 1013893 by allowing the setting of the iptables drop
action with a configuration flag. It is expected that this would be
used for run a LOGDROP action before actually dropping the packet.

DocImpact: the drop action for iptables rules can now be configured
for nova-network users with the iptables_drop_action flag.

Change-Id: I15720d2742955611929a4d7181a269795296e025
Merge "Make iptables drop action configurable."