Currently, log extension is only compatible with ovs agent [1].
In order to support security group logging based iptables, this
patch will make log extension compatible with linuxbridge agent,
too.
[1] https://bugs.launchpad.net/neutron/+bug/1743463
Change-Id: Ic6528ac3ecfd9fd2dac6f2b8233df5554a867520
Partially-implements: blueprint security-group-logging