Add support for oslo messaging notifications in Keystone charm

This change introduces a new configuration option `enable-oslo-notify`
to the Keystone charm, which allows users to enable `oslo_messaging_notifications`.
This is necessary for services like Fleio that require Keystone to send notifications.

If `enable-oslo-notify` is set to `True`, the following section is added
to the `keystone.conf`:
[oslo_messaging_notifications] driver = messagingv2

By default, `enable-oslo-notify` is set to `False`, keeping the existing behavior
unchanged unless explicitly configured.

Closes-Bug: #2085335
Change-Id: I0b294f0feea3cac25492e6a40fa662c81819f61e

config.yaml

@@ -477,3 +477,11 @@ options:
       this option sets True as the default value, which is consistent with the
       default value 'WSGISocketRotation On' in Apache. This option should be
       used with caution. Please read the Apache doc page for more information.
+  enable-notifications:
+    type: boolean
+    default: False
+    description: |
+      Enables `oslo.messaging` notifications in Keystone.
+      When set to `True`, Keystone will be configured to send notifications
+      using the `messagingv2` driver. The default value is `False`,
+      meaning notifications are disabled unless explicitly enabled.

hooks/keystone_context.py

@@ -270,6 +270,8 @@ class KeystoneContext(context.OSContextGenerator):
 
         ctxt['default_authorization_ttl'] = config('default-authorization-ttl')
 
+        ctxt['enable_notifications'] = config('enable-notifications')
+
         return ctxt
 
     ALLOWED_SECURITY_COMPLIANCE_SCHEMA = {
@@ -410,6 +412,16 @@ def fernet_enabled():
         return True
 
 
+def notifications_enabled():
+    """Helper function for determining whether notifications are enabled.
+
+    :returns: True if notifications are enabled.
+    :rtype: bool
+    """
+
+    return config('enable-notifications')
+
+
 class KeystoneFIDServiceProviderContext(context.OSContextGenerator):
     interfaces = ['keystone-fid-service-provider']
 

templates/rocky/keystone.conf

@@ -130,3 +130,8 @@ admin_project_name = admin
 # Bug #1819134
 max_request_body_size = 114688
 
+{% if enable_notifications -%}
+[oslo_messaging_notifications]
+driver = messagingv2
+{% endif -%}
+

unit_tests/test_keystone_contexts.py

@@ -247,6 +247,18 @@ class TestKeystoneContexts(CharmTestCase):
         result = context.fernet_enabled()
         self.assertTrue(result)
 
+    def test_notifications_enabled_yes(self):
+        self.os_release.return_value = 'rocky'
+        self.test_config.set('enable-notifications', True)
+        result = context.notifications_enabled()
+        self.assertTrue(result)
+
+    def test_notifications_enabled_no(self):
+        self.os_release.return_value = 'rocky'
+        self.test_config.set('enable-notifications', False)
+        result = context.notifications_enabled()
+        self.assertFalse(result)
+
     @patch.object(context, 'relation_ids')
     @patch.object(context, 'related_units')
     @patch.object(context, 'relation_get')