The local vlan tag of openvswitch port become 4095 when dynamic segment is released

[Huang Cheng]

env: OpenStack Ocata, ml2 hierarchical port binding with openvswitch as bottom driver

When the bottom segment already bound to a l2 driver is dynamiclly released,
the associated neutron.ml2_port_binding_levels.segment_id will be set "NULL",
and cause the local vlan tag of openvswitch port become 4095 (aka "dead vlan")
after an openvswitch restart.

The foreign key constraint:
|------------------------------------+--------------------+-----------|
| foreign key | reference | on delete |
|------------------------------------+--------------------+-----------|
| ml2_port_binding_levels.segment_id | networksegments.id | set null |
|------------------------------------+--------------------+-----------|