This commit adds common_agent_extension class which is agent API
for L2 extension drivers used e.g. by Linuxbridge agent.
This is necessary to be able to use instance of iptables_manager
used in firewall driver also in L2 extension drivers (like qos).
This patch refactors little bit iptables_manager code to make possible
to initialize e.g. mangle or nat table on demand, even if iptables
is created as "state_less"
Change-Id: I3b66e49b7f176124e8aea3eb96d0d465f1ab1ea0
Closes-Bug: #1736674
cbee0f9f88