OFAgent: Avoid processing ports which are not yet ready

Port the following patch to OFAgent.
commit: 2702baed390d094b0eac07d0ae167ed236868d00
https://review.openstack.org/#/c/65838/

Closes-Bug: 1307745

Change-Id: Ie1fe0944fb541ba45ffb5b03747d5da289cede51
(cherry picked from commit b7a753ce80d34f7d98c4b401e364b5f6a740c3f4)
Merge "OFAgent: Avoid processing ports which are not yet ready" into stable/icehouse