Add 'assigned_ips' field to networks for dhcp assignment

Signed-off-by: Jon Ludlam <jonathan.ludlam@citrix.com>
(cherry picked from commit ae610a9e82f0f7bf640ef446a039d7434a76d9b9)
Merge pull request #1839 from johnelse/assigned-ips-xs64bit

Add 'assigned_ips' field to networks for dhcp assignment