Fix nic order not correct after reboot

Previously if an instance was rebooted or migrated the original nic order
would not be maintained. This patch retrieves the correct nic order from
info_cache so that the order is mantained.

Fixes bug 1187092

Change-Id: I3f5488cc14c692fde7d5bd3376389e2f2f5550ff

Conflicts:
	nova/tests/network/test_quantumv2.py