OFAgent: Improve handling of security group updates

Port the following patch to OFAgent.
commit: 3046c4ae22b10f9e4fa83a47bfe089554d4a4681
https://review.openstack.org/#/c/63100/

Partial-Bug: 1293265

Change-Id: Iecdfee894ff2fd5f05f63f040dd70821af124737
(cherry picked from commit 6e46769594c856647b0f9ab0dddbe6dd9c9163a1)
Merge "OFAgent: Improve handling of security group updates" into stable/icehouse