Clean routers in DvrRoutersNegativeTestExtended

It completes the initial patch "Add tempest tests for DVR router state
management" which doesn't clean 2 routers [1].

[1] https://review.opendev.org/#/c/668760/

Change-Id: Iec14343578ca587143a55bbd3d602790730d31b6
Signed-off-by: Cédric Ollivier <ollivier.cedric@gmail.com>
diff --git a/neutron_tempest_plugin/api/test_routers_negative.py b/neutron_tempest_plugin/api/test_routers_negative.py
index f085fc9..5f24732 100644
--- a/neutron_tempest_plugin/api/test_routers_negative.py
+++ b/neutron_tempest_plugin/api/test_routers_negative.py
@@ -90,9 +90,9 @@
         # create a centralized router
         router_args = {'tenant_id': self.client.tenant_id,
                        'distributed': False}
-        router = self.admin_client.create_router(
+        router = self._create_admin_router(
             data_utils.rand_name('router'), admin_state_up=True,
-            **router_args)['router']
+            **router_args)
         self.assertTrue(router['admin_state_up'])
         self.assertFalse(router['distributed'])
         # attempt to set the router to distributed, catch BadRequest exception
@@ -107,9 +107,9 @@
         # create a centralized router
         router_args = {'tenant_id': self.client.tenant_id,
                        'distributed': False}
-        router = self.admin_client.create_router(
+        router = self._create_admin_router(
             data_utils.rand_name('router'), admin_state_up=True,
-            **router_args)['router']
+            **router_args)
         self.assertTrue(router['admin_state_up'])
         self.assertFalse(router['distributed'])
         # take the router down to modify distributed->True