Port "Accept is_public=None .." to v3 tree

The following commit(I2a9b1c24cd744c8819c1d41e0ad103d05398401c) have
not been merged to v3 tree now, because the commit was merged to the
existing api after porting flavors.py to v3 tree.

    commit 645c2d3a06bbf8b0854ebcc539ad33dd706866c5
    Author: Xavier Queralt <xqueralt@redhat.com>
    Date:   Wed Jun 12 14:59:29 2013 +0200

        Accept is_public=None when listing all flavors

This patch ports the commit to v3 tree.

Partially implements blueprint nova-v3-api

Change-Id: Id1ff1366c16f0caac50e279b2add5ec9d9b9bf5f
Merge "Port "Accept is_public=None .." to v3 tree"