Deprecate l3-agent router_id option

L3-agent router_id option has been defined in order to associate a
l3-agent to a specific router when use_namespaces=False. The option
use_namespaces has been removed in Mitaka, so router_id option is no
longer needed and can be deprecated in order to prepare its removal in N
cycle.

Closes-Bug: #1518776
Change-Id: Id79499ccbf3ea69ca4cd7e4b2f812f6dbc0406e0

etc/l3_agent.ini

@@ -20,6 +20,7 @@
 
 # If non-empty, the l3 agent can only configure a router that has the matching
 # router ID.
+# This option is deprecated and will be removed in the N release.
 # router_id =
 
 # When external_network_bridge is set, each L3 agent can be associated

neutron/agent/l3/config.py

@@ -49,6 +49,7 @@ OPTS = [
                help=_("Send this many gratuitous ARPs for HA setup, if "
                       "less than or equal to 0, the feature is disabled")),
     cfg.StrOpt('router_id', default='',
+               deprecated_for_removal=True,
                help=_("If non-empty, the l3 agent can only configure a router "
                       "that has the matching router ID.")),
     cfg.BoolOpt('handle_internal_only_routers',

releasenotes/notes/deprecate-router_id-34aca9ea5ee9e789.yaml

@@ -0,0 +1,5 @@
+---
+prelude: >
+    Deprecate router_id option in order to prepare its removal in N cycle.
+upgrade:
+  - router_id option deprecation