Fix for bug 676.

Ensure that when a neutron vRouter has the external gateway interface
removed, the underlying egw interface is removed

(cherry picked from commit 4442b7aec92c212640744866913f06ef1d282efb)

Change-Id: I855b13dd1cfff472783774d13cad98d48b68efd1
Signed-off-by: Ryan Moats <rmoats@us.ibm.com>