Bug 1730: L3: Trigger processing of neutron port updates based on events from southbound

Bug 1730: L3: Trigger processing of neutron port updates based on events from southbound

When interface events originate from the southbound side of ovsdb, the L3 forwarding
feature in ovsdb expects to be notified. W/out this code, flows needed to program L3
functionality will get skipped.

Due to my mistake, the logic for connecting the southbound event to the
already implemented neutron port handler is missing. This bug is created
to account for the the small code missing in L3 handler; past the code
freeze.

Change-Id: I7d124a66425352dd4690ab830cc29d0b94a87e67
Signed-off-by: Flavio Fernandes <ffernand@redhat.com>