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_mtu.py b/neutron_tempest_plugin/scenario/test_mtu.py
index 29ccb01..6f4fd80 100644
--- a/neutron_tempest_plugin/scenario/test_mtu.py
+++ b/neutron_tempest_plugin/scenario/test_mtu.py
@@ -46,24 +46,12 @@
             secgroup_id=cls.secgroup['security_group']['id'])
         cls.create_pingable_secgroup_rule(
             secgroup_id=cls.secgroup['security_group']['id'])
-        if CONF.neutron_plugin_options.default_image_is_advanced:
-            cls.use_advanced_image = False
-            cls.username = CONF.validation.image_ssh_user
-        else:
-            cls.use_advanced_image = True
-            cls.username = CONF.neutron_plugin_options.advanced_image_ssh_user
+        cls.setup_advanced_image()
 
     def create_pingable_vm(self, net, keypair, secgroup):
-        if self.use_advanced_image:
-            flavor_ref = CONF.neutron_plugin_options.advanced_image_flavor_ref
-            image_ref = CONF.neutron_plugin_options.advanced_image_ref
-        else:
-            flavor_ref = CONF.compute.flavor_ref
-            image_ref = CONF.compute.image_ref
-
         server = self.create_server(
-            flavor_ref=flavor_ref,
-            image_ref=image_ref,
+            flavor_ref=self.flavor_ref,
+            image_ref=self.image_ref,
             key_name=keypair['name'],
             networks=[{'uuid': net['id']}],
             security_groups=[{'name': secgroup[