Add flavor-manage to core for V3 API

Adds the flavor-manage plugin to the core list for the V3 API.
This is part of the process of adding all of the flavor related
extensions to core, with the exception of rxtx, as per the discussion
here: https://etherpad.openstack.org/p/NovaV3APICore

Partially implements blueprint nova-v3-api

DocImpact

Change-Id: Idbedce5cc5936abc52235cd40573c8fe841f24fe
Merge "Add flavor-manage to core for V3 API"