Port "Make flavors is_public option .." to v3 tree

The following commit(I5b37fa0bb19683fe1642fd81222547d4a317054e) 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 b65d506a5f9d9b2b20777a9aceb44a8ffed6a5de
    Author: Russell Bryant <rbryant@redhat.com>
    Date:   Thu Jun 27 21:00:05 2013 +0000

        Make flavors is_public option actually work

This patch ports the commit to v3 tree.

Partially implements blueprint nova-v3-api

Change-Id: I3c7f11c53594118456d0cd62eb1ddf74634b77c3
Merge "Port "Make flavors is_public option .." to v3 tree"