Update send_arp_for_ha default in l3_agent.ini

The default for send_arp_for_ha was 3 and is now 0.
This patch changes the comment in l3_agent.ini to
show the new default.

Change-Id: I77370350ffee6ad7b94aa7db341c062ebc0b7d89
Merge "Update send_arp_for_ha default in l3_agent.ini"