Sync CDM among Decision Engines by using notification pool

This commit allows to consume notifications via notifications pools[1].
Listeners in notification pools recieves a copy of notification. It
will let Watcher to sync Data Models of Decision Engines.

[1]: https://docs.openstack.org/oslo.messaging/ocata/notification_listener.html

Change-Id: Ie37528263181924f84510500fc1277b0237c1df8
Partially-Implements: blueprint support-watcher-ha-active-active-mode

.zuul.yaml

@@ -73,7 +73,7 @@
                 live_migration_uri: 'qemu+ssh://root@%s/system'
         devstack_services:
           watcher-api: false
-          watcher-decision-engine: false
+          watcher-decision-engine: true
           watcher-applier: false
           # We need to add TLS support for watcher plugin
           tls-proxy: false