OFAgent: Process port_update notifications in the main agent loop

Port the following patch to OFAgent.
commit: 5e6e592132aa9a98936ce3bfdb66efc7832caafb
https://review.openstack.org/#/c/61964/

Partial-Bug: 1293265

Conflicts:
	neutron/plugins/ofagent/agent/ofa_neutron_agent.py

Change-Id: I53813d12c66dc746cd373fd91ff9bd9bdbf222db
(cherry picked from commit 34e3be16a196c3da32cd6fa6147f6e00636a9aaf)
Merge "OFAgent: Process port_update notifications in the main agent loop" into stable/icehouse