Network: ensure that ports are 'unset' when instance is deleted

Commit a141206e9dfd31955b9b31d9e5a7f73bbd8510ca ensured that user defined
ports were not deleted when an instance is deleted or rescheduled. The
problem with this commit is that it did not reset the 'device_id' and
'device_owner' of the port. This leads to port leakage and eventually a tenant
is unable to launch any instances as their port quota has exceeded the
allocated amount.

The solution is for the deallocation to reset the 'device_id' and the
'device_owner' of the port. This is allocated when the port is actually assigned
to an instance.

In the case of minesweeper (which runs on OpenStack), the tenant was leaking ports.
The root cause was that the user defined ports were assigned to a specific instance,
which was deleted, as a result of a failed reschedule (which is legitimate).

When an instance is deleted all artificats of that instance should be reset on the
port.

Change-Id: Ie1516c299ce6d79faf2d5103a14d6d8931d3ede2
Closes-bug: #1321381
(cherry picked from commit 803d59e9f409a59403b2ce55040acc02e14eee28)

Conflicts:
	nova/tests/network/test_neutronv2.py