Server Pools Storage

 This partially implements the changes needed
 for server pools. Done is:
 - Added pools table
 - Added pool_attributes table
 - Added pool CRUD calls in central and storage
   with unit tests
 - Created default pool

 Still to do in separte patchset(s):
 - Add pool_id column to domains table and change
   necessary code
 - Migrate servers from servers table pool_
   attributes table and change necessary code
 - Remove all old server API code

 Partially-implements: blueprint server-pools-storage

Change-Id: I55cd2fab94cef0e72d4f6cac2b24734f24390711