OFAgent: Avoid re-wiring ports unnecessarily

Port the following patch to OFAgent.
commit: a9050757d94009c68f06c9f374de33eefbf06aa9
https://review.openstack.org/#/c/63558/

Partial-Bug: 1293265

Change-Id: I35d58bad104b01136c2502c933475ac85a3752a5
(cherry picked from commit fcdee4d44558b6d1ae647f38b2584820352115de)
Merge "OFAgent: Avoid re-wiring ports unnecessarily" into stable/icehouse