Fix for bug 677

Ensure that when a neutron router with external_gateway_info
set is deleted that the underlying GW address is removed.

(cherry picked from commit e664f81ef6565f780a436717b322ea79cdbda9af)

Change-Id: I8c7da2bc31e72aba37276fe88e3dd7f79562acea
Signed-off-by: Ryan Moats <rmoats@us.ibm.com>