Replace the usage of 'admin_manager' with 'os_admin'

In tempest, alias 'admin_manager' has been moved to 'os_admin'
'alt_manager' to 'os_alt' and 'manager' to 'os_primary' in
version Pike, and it will be removed in version Queens[1].


[1]I5f7164f7a7ec5d4380ca22885000caa0183a0bf7

Closes-bug: 1697588

Change-Id: Ib41a1bcff70cc7323b90a222c9f64d66e0203e53
diff --git a/neutron/tests/tempest/scenario/test_migration.py b/neutron/tests/tempest/scenario/test_migration.py
index db89716..4951c61 100644
--- a/neutron/tests/tempest/scenario/test_migration.py
+++ b/neutron/tests/tempest/scenario/test_migration.py
@@ -32,7 +32,7 @@
         super(NetworkMigrationTestBase, cls).skip_checks()
 
     def _check_update(self, router, is_dvr, is_ha):
-        router = self.admin_manager.network_client.show_router(router['id'])
+        router = self.os_admin.network_client.show_router(router['id'])
         self.assertEqual(is_dvr, router['router']['distributed'])
         self.assertEqual(is_ha, router['router']['ha'])
 
@@ -44,13 +44,13 @@
         self.setup_network_and_server(router=router)
         self._check_connectivity()
 
-        self.admin_manager.network_client.update_router(
+        self.os_admin.network_client.update_router(
             router_id=router['id'], admin_state_up=False)
-        self.admin_manager.network_client.update_router(
+        self.os_admin.network_client.update_router(
             router_id=router['id'], distributed=after_dvr, ha=after_ha)
         self._check_update(router, after_dvr, after_ha)
 
-        self.admin_manager.network_client.update_router(
+        self.os_admin.network_client.update_router(
             router_id=router['id'], admin_state_up=True)
         self._check_connectivity()