Let devstack automatically pick FLAT_INTERFACE

The FLAT_INTERFACE was previously incorrectly being set to br100,
but was being ignored by nova. However nova recently fixed a bug
(I92ee96e3aa34f7cd154a8ce4b33852b96cabe8c9) and now this is being
checked, resulting in "NovaException: Failed to add interface: device
br100 is a bridge device itself; can't enslave a bridge device to
a bridge device." when a nova instance is being provisioned.

This was causing an issue with running int-tests, since we were
unable to spin up nova instances.

Change-Id: Ifb78978693c7d646fe357fe8979606468c945653