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/base.py b/neutron_tempest_plugin/scenario/base.py
index f7d08eb..36d15d8 100644
--- a/neutron_tempest_plugin/scenario/base.py
+++ b/neutron_tempest_plugin/scenario/base.py
@@ -269,6 +269,18 @@
         raise cls.skipTest("No L3 agent with '%s' extension enabled found." %
                            extension)
 
+    @classmethod
+    def setup_advanced_image(cls):
+        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
+
     @removals.remove(version='Stein',
                      message="Please use create_floatingip method instead of "
                              "create_and_associate_floatingip.")