Apply Ironic's server-ip-address as TFTP next-server

This patch uses existing DHCP option 255 (server-ip-address)
provided by Ironic and applies it as next-server for the purpose
of provisioning a baremetal server with OVN DHCP.

Related-Bug: #2007167
Change-Id: I59038639a8411c11c5fb8b366d9c858ef3db4f70
(cherry picked from commit dbfc18d)

Author: busterswt

neutron/common/ovn/constants.py

@@ -166,7 +166,8 @@
         '119': 'domain_search_list',
         '252': 'wpad',
         '210': 'path_prefix',
-        '150': 'tftp_server_address'},
+        '150': 'tftp_server_address',
+        '255': 'next_server'},
     6: {'server-id': 'server_id',
         'dns-server': 'dns_server',
         'domain-search': 'domain_search',