Enable send_arp_for_ha in l3_agent.ini

send_arp_for_ha is defined for nova networks in the nova cookbook and 
referenced here but never templated for quantum/neutron l3_agent.

- Add attribute for send_arp_for_ha_count in nova networks
- Add attribute for send_arp_for_ha in quantum/neutron
- Document new attributes in README
- Template send_arp_for_ha in l3_agent.ini

https://github.com/rcbops-cookbooks/nova will need to be updated following
this change.

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