Port flavors core API to v3 tree
Port flavors core API to v3 tree

Port flavors core API (which does *not* include all extensions, like flavor
create) to the V3 API, adapting the way it's loaded to fit with the new
extension framework and, of course, its tests.

This also change the flavors requests and responses links on unit tests since
project_id  is no longer present on v3 url

Partially implements blueprint v3-api-core-as-extensions

Change-Id: Ide2eb22a89be3bd9adcf5e09cbef23e824a83fc1

n0ano/gantt,n0ano/gantt