OFAgent: Fixing lost vlan ids on interfaces

Port the following patch to OFAgent.
commit: 60cb0911712ad11688b4d09e5c01ac39c49f5aea
https://review.openstack.org/#/c/66375/

Closes-Bug: 1309286

Change-Id: I04f99ef7928cd07959b29eb64b5744df397979b8