Prevent allowed-address-pair port-update commands

This patch is a forward-port of a patch from puppet-coe-service-patch
which had the following commit message:

---
commit 98f45411c5305c297881d6477e5b4b6ac121d19d
Author: Kevin Bringard <kevinbri@cisco.com>
Date:   Thu Jun 19 11:37:17 2014 -0600

    Prevent allowed-address-pair port-update commands on shared networks

    We have to check for dhcp agents as they need to update the allowed
    address pairs, and functionality will break if we don't allow for it.

    This code is currently running in SVL2 and is being tested by SDU

    Change-Id: Id4f9ee93b9e5efd147f176b7aa76f4e4206c48e
---

Implements: rally-user-story US4145
Implements: rally-task TA3473
Not-in-upstream: true