plugin/ryu: make live-migration work with Ryu plugin

plugin/ryu: make live-migration work with Ryu plugin

Fixes bug 1085861

Live-migration with Ryu plugin doesn't work because the unique constraint of
PortBinding table is violated as follows.
Now Ryu can handle those informations itself, so remove the table itself and
simplify Ryu plugin.

> ERROR [quantum.api.v2.resource] update failed
> Traceback (most recent call last):
>   File "/quantum/api/v2/resource.py", line 95, in resource
>     result = method(request=request, **args)
>   File "/quantum/api/v2/base.py", line 397, in update
>     obj = obj_updater(request.context, id, **kwargs)
>   File "/quantum/plugins/ryu/ryu_quantum_plugin.py", line 226, in update_port
>     port_binding.dpid, port_binding.port_no))
> InvalidInput: Invalid input for operation: invalid (datapath_id, port_no) requested (00002eab88ec5140, 4), but (0000c2f19014c74a, 1).

Change-Id: I7d993fd01125a27f6bf8e1b3fcac79ddcb8cb361