Add precommit and postcommit support for security group in ML2

Neutron has already be able to make the agentless driver receive the BEFORE/AFTER event by neutron callback logic[1]. But callback logic has a flaw, that it has no ordering guarantee[2]. It works in most of the agentless cases, but when there is sync issue between neutron DB and the controller DB which mech driver leverages, order becomes important. For example, the update of the default security group rules would only work on the following created ports.

This spec will add new SecuritygroupContext, like network/port/subnet. And add related methods in ML2 by wrap the current security group class, with mech driver pre/post commit call.

[1] https://bugs.launchpad.net/neutron/+bug/1444112
[2] http://docs.openstack.org/developer/neutron/devref/callbacks.html