Enable send_arp_for_ha_count in nova.conf

- Add attribute for send_arp_for_ha_count in nova conf provider to match
  functionality in quantum/neutron l3_agent.ini

See https://github.com/rcbops/chef-cookbooks/issues/877

(cherry picked from commit 00ae912614f9e2691e64639f82efe942183f9bbe)