Quantumapi returns an empty network list

When nova retries network list and if the list is empty, quantumapi
should return a empty list instead of an empty dictionnary.

Fixes bug 1189976

Conflicts:

	nova/tests/network/test_quantumv2.py

Change-Id: I5b8c498a6c86e92d0de14d8de9e55722408c5669
(cherry picked from commit 14d3ae58c1ec39edd1b0b2a2a1947c5f8e96bab5)