Use tempest.lib tenants_client

Tempest 12.1.0 has been released for new library interfaces.
These interfaces are stable[1] and we need to switch using them
instead of current using unstable interfaces.
This will be helpful to avoid breaking gate jobs.

[1]: https://github.com/openstack/tempest/blob/master/doc/source/library.rst#stability

Change-Id: I74a5d5ed43f758b88a22ded293d85d046005d41f
diff --git a/neutron/tests/tempest/api/clients.py b/neutron/tests/tempest/api/clients.py
index 8b51c6e..c6f41d0 100644
--- a/neutron/tests/tempest/api/clients.py
+++ b/neutron/tests/tempest/api/clients.py
@@ -15,8 +15,8 @@
 
 from tempest.lib.services.compute import keypairs_client
 from tempest.lib.services.compute import servers_client
+from tempest.lib.services.identity.v2 import tenants_client
 from tempest import manager
-from tempest.services.identity.v2.json import tenants_client
 
 from neutron.tests.tempest import config
 from neutron.tests.tempest.services.network.json import network_client