Fixes bug 1045601.
When attempting to create a flat network on a physical_network on
which a flat network is already allocated, the openvswitch and
linuxbridge plugins return a FlatNetworkInUse exception rather than a
VlanIdInUse exception referencing VLAN -1.
Change-Id: Id79e917e1376ebbb199efc3f14d9f378c6a04050