Fix one port can be attached to more devices

a port attached to some device should not be attached to another device again

Fixes: bug #1204850
(cherry picked from commit c9c634150f740d2d2c2146c8e6d583d73d8b587c)

Conflicts:
	nova/network/quantumv2/api.py
	nova/tests/network/test_quantumv2.py

Change-Id: I60de0fa265358ac5b5e1b2c8f370862b32f9e724
Merge "Fix one port can be attached to more devices" into stable/grizzly