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
Revert "Hide ipv6 subnet API attributes"

This reverts commit d739486a3f70cb83f8730cf7201a8b8ccf4b4c70.

Change-Id: I27352ba5442da8c69c2d1ab6d5e333e95dc5ebf7
Merge "Revert "Hide ipv6 subnet API attributes""
Merge "OFAgent: Avoid re-wiring ports unnecessarily"