Correct invalid client in ServersNegativeTestMultiTenantJSON

In ServersNegativeTestMultiTenantJSON's setUp, self.client
should be self.servers_client, otherwise we will get:
<AttributeError: "'ServersNegativeTestMultiTenantJSON' object
has no attribute 'client'">, and so we will encounter twice
unnecessary rebuild_server.

Change-Id: Ie5559bd4f1cbbb0b128946c21bd737585f79ffad
diff --git a/tempest/api/compute/servers/test_servers_negative.py b/tempest/api/compute/servers/test_servers_negative.py
index a480a15..7943b37 100644
--- a/tempest/api/compute/servers/test_servers_negative.py
+++ b/tempest/api/compute/servers/test_servers_negative.py
@@ -551,7 +551,7 @@
     def setUp(self):
         super(ServersNegativeTestMultiTenantJSON, self).setUp()
         try:
-            waiters.wait_for_server_status(self.client, self.server_id,
+            waiters.wait_for_server_status(self.servers_client, self.server_id,
                                            'ACTIVE')
         except Exception:
             self.__class__.server_id = self.rebuild_server(self.server_id)