Use configured overrides for internal/external
Use configured overrides for internal/external

Some clouds (hi internap) have very difficult to discover internal and
external networks. They defy all of our other logic, but we have
configuation for this purpose. However, we were lettting discovery
override configuration. Boo on us.

Change-Id: I62e34b1f0f32c19acbeccd11bc2ea38a2b119951

dtroyer/python-openstacksdk,openstack-infra/shade,openstack/python-openstacksdk,stackforge/python-openstacksdk,openstack/python-openstacksdk,stackforge/python-openstacksdk,dtroyer/python-openstacksdk,openstack-infra/shade