Add pool_id column to Domains Table

 This adds the pool_id column to the domains table
 along with the necessary code changes and tests as
 part of all the changes that need to be done to
 accomodate server pools

 Partially-implements: blueprint server-pools-storage

 - Migrate servers from servers table to
   poolattributes table
 - Remove all old server api code
 - Make changes to backends to use poolattribute
   nameservers instead of servers

Change-Id: I535a04cac27d1068398fd982ca14016c38cbd134
Merge " Add pool_id column to Domains Table"