Add a spec for refactoring datasource drivers
Add a spec for refactoring datasource drivers

This spec describes how we can refactor datasource drivers to make it easier
to write new datasource drivers, and reduce duplicate code.

blueprint: refactor-drivers

Change-Id: I81c302883b0865975b211331899b3219739dfde9

zhang935602068/congress-specs,ekcs/congress-specs,openstack/congress-specs