Deprecate send_arp_for_ha option

It puzzles me why we would want to have it configurable. Having it = 0
is just plain bad (it breaks a floating IP roaming around HA routers),
having it = 1 may be unsafe if clients miss the update, having it more
than 3 (the default) is probably wasteful. That makes me think that
maybe we should not have it in the first place.

The patch that introduced the option also introduced the feature itself,
and does not provide any clue around why we would need it:
I125dbc57b90027dc5e99ff0a5d6877843a0b02a5

Maybe the option is in the tree because, in Assaf Muller's words, "we're
a bunch of lazy developers that like to shift the responsibility to our
poor users that have to deal with thousands of configuration options".

I suggest we just move with deprecation and removal here.

Change-Id: I9d12b8f4c25ddf91312153f236915c0c14302e2d
Related-Bug: #1639879

neutron/conf/agent/l3/config.py

@@ -42,6 +42,7 @@ OPTS = [
                 default=9697,
                 help=_("TCP Port used by Neutron metadata namespace proxy.")),
     cfg.IntOpt('send_arp_for_ha',
+               deprecated_for_removal=True,
                default=3,
                help=_("Send this many gratuitous ARPs for HA setup, if "
                       "less than or equal to 0, the feature is disabled")),

releasenotes/notes/deprecate-send_arp_for_ha-0281853632f58e8d.yaml

@@ -0,0 +1,5 @@
+---
+deprecations:
+  - L3 agent send_arp_for_ha configuration option is deprecated and will be
+    removed in Pike. The functionality will remain and the agent will send
+    three gratuitious ARPs whenever a new floating IP is configured.