Add flavor-extra-specs to core for V3 API

Adds the flavor-extra-specs 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

Change-Id: Id4b4fad5e584bdf2bbbaaae579a07e28c312ef34
Merge "Add flavor-extra-specs to core for V3 API"