Iterate over same port_id if more than one exists

In certain cases where multiple ports can have the same
external_ids:iface_id property, the ovs agent will arbitrarily choose
one and ignore the rest. In case the chosen port isn't on the
integration bridge the ovs agent is managing, an error is returned to
the calling function. This is faulty since one of the other ports may
belong to the correct bridge and it should be chosen instead.

This is interesting for future L3 HA integration tests, where 2
different instances of l3 agents are needed to run on the same machine.
In this case, both agents will register ports which have the same
iface_id property, but obviously only one of the ports is relevant for
each agent.

Closes-bug: #1370914
Related-bug: #1374947
Change-Id: I05d70417e0f78ae51a9ce377fc93a3f12046b313
Merge "Iterate over same port_id if more than one exists"