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_dvr.py b/neutron/tests/tempest/scenario/test_dvr.py
index 049aa8f..0b03276 100644
--- a/neutron/tests/tempest/scenario/test_dvr.py
+++ b/neutron/tests/tempest/scenario/test_dvr.py
@@ -40,7 +40,7 @@
         port_id = self.client.list_ports(
             network_id=network_id,
             device_owner=constants.DEVICE_OWNER_ROUTER_SNAT)['ports'][0]['id']
-        self.admin_manager.network_client.update_port(
+        self.os_admin.network_client.update_port(
             port_id, admin_state_up=False)