Fix volume transfer tests with tenant isolation disabled

This commit fixes the volume transfer tests for when tenant isolation
is disabled. The volume transfer tests still had a reference to the old
auth style to get the tenant name if tenant isolation was disabled.
The recent auth_provider refactor didn't update this call so the test
would fail. This commit updates it to use the new credential dict to
get the tenant_name

Change-Id: Ie7fe39baf6f62fa6966631d7890891252602ed8d
diff --git a/tempest/api/volume/test_volume_transfers.py b/tempest/api/volume/test_volume_transfers.py
index fc4f07d..12160e3 100644
--- a/tempest/api/volume/test_volume_transfers.py
+++ b/tempest/api/volume/test_volume_transfers.py
@@ -47,7 +47,7 @@
                                          interface=cls._interface)
         else:
             cls.os_alt = clients.AltManager()
-            alt_tenant_name = cls.os_alt.tenant_name
+            alt_tenant_name = cls.os_alt.credentials['tenant_name']
             identity_client = cls._get_identity_admin_client()
             _, tenants = identity_client.list_tenants()
             cls.alt_tenant_id = [tnt['id'] for tnt in tenants