Accept is_public=None when listing all flavors

nova-client uses a None (with capital N) when listing all the flavors.
Right now this is interpreted as a wrong query by n-api preventing us
from listing the private flavors using the nova-client.

Fixes bug #1190239

Change-Id: I2a9b1c24cd744c8819c1d41e0ad103d05398401c
Merge "Accept is_public=None when listing all flavors"