Add a new method ha_state_change to L3 agent extension

This is needed by VPNaaS agent extension and other advanced
services in case they support L3 HA router.

Change-Id: Idaa0e5914147f7cdc9966d022ac5c6ab0165966a
Depends-On: I70060a6cf129f9dbeae2bd100a64d7358f84e99a

neutron_lib/agent/l3_extension.py

@@ -51,3 +51,13 @@ class L3AgentExtension(extension.AgentExtension):
         :param context: RPC context.
         :param data: Router data.
         """
+
+    @abc.abstractmethod
+    def ha_state_change(self, context, data):
+        """Change router state from agent extension.
+
+        Called on HA router state change.
+
+        :param context: rpc context
+        :param data: dict of router_id and new state
+        """

releasenotes/notes/l3-agent-extensions-ha-state-change-837140efe4187a99.yaml

@@ -0,0 +1,4 @@
+---
+features:
+  - |
+    A new abstract method ``ha_state_change`` has been added to ``L3AgentExtension``.