Add common method for advanced image configuration

Add setup_advanced_image() to BaseTempestTestCase and update tests
to use it.

Related-Bug: OSPRH-19617
Assisted-by: Claude Code Sonnet 4.5

Change-Id: I4abea05a565061a57f71c37b98a101129cb388e2
Signed-off-by: Renjing Xiao <rxiao@redhat.com>
diff --git a/neutron_tempest_plugin/scenario/test_multicast.py b/neutron_tempest_plugin/scenario/test_multicast.py
index a28328b..4d79f85 100644
--- a/neutron_tempest_plugin/scenario/test_multicast.py
+++ b/neutron_tempest_plugin/scenario/test_multicast.py
@@ -150,15 +150,7 @@
     def resource_setup(cls):
         super(BaseMulticastTest, cls).resource_setup()
 
-        if CONF.neutron_plugin_options.default_image_is_advanced:
-            cls.flavor_ref = CONF.compute.flavor_ref
-            cls.image_ref = CONF.compute.image_ref
-            cls.username = CONF.validation.image_ssh_user
-        else:
-            cls.flavor_ref = (
-                CONF.neutron_plugin_options.advanced_image_flavor_ref)
-            cls.image_ref = CONF.neutron_plugin_options.advanced_image_ref
-            cls.username = CONF.neutron_plugin_options.advanced_image_ssh_user
+        cls.setup_advanced_image()
 
         # setup basic topology for servers we can log into it
         cls.network = cls.create_network()