Change tempest Conflict exc to tempest-lib exc

This commit changes tempest.exceptions.Conflict to
tempest_lib.exceptions.Conflict. This is one of the migrating
rest client to tempest-lib works.

Change-Id: I860a961c442f35ef4a75013e19bfe92f61a587b8
diff --git a/tempest/api/compute/admin/test_servers_negative.py b/tempest/api/compute/admin/test_servers_negative.py
index 9800bb4..81dabb1 100644
--- a/tempest/api/compute/admin/test_servers_negative.py
+++ b/tempest/api/compute/admin/test_servers_negative.py
@@ -144,6 +144,6 @@
         self.assertEqual(202, resp.status)
         self.client.wait_for_server_status(server_id, 'SUSPENDED')
         # migrate an suspended server should fail
-        self.assertRaises(exceptions.Conflict,
+        self.assertRaises(lib_exc.Conflict,
                           self.client.migrate_server,
                           server_id)