Relying on 'external_network_bridge=br-ex' for the L3
agent has been deprecated in Neutron. This patch adjusts
the devstack defaults to setup Neutron in the preferred
manner (empty external_network_bridge value and
correct bridge_mappings for the L2 agent).
This will also help with correct MTU calculations now that
the external network will have the correct segmentation
type on it ('flat' now instead of 'vxlan' by default).
Related-Bug: #1511578
Related-Bug: #1603493
Change-Id: Id20e67aba5dfd2044b82c700f41c6e648b529430