Do not use class variable for clients in ContainerSyncTest

Running tempest.api.object_storage.test_container_sync_middleware.
ContainerSyncMiddlewareTest.test_container_synchronization along with
all other swift testcases will fail with Unauthorized error, but it can
pass when being executed alone.

This is because clients in parent class ContainerSyncTest is a class
variable, which will cause trouble when using dynamic credentials. So
this is to change clients to an instance variable.

Change-Id: I10f04fadae95932258a5df58ef68d80f81075b79
Closes-Bug: #1740075

tempest/api/object_storage/test_container_sync.py

@@ -33,8 +33,6 @@
 
 
 class ContainerSyncTest(base.BaseObjectTest):
-    clients = {}
-
     credentials = [['operator', CONF.object_storage.operator_role],
                    ['operator_alt', CONF.object_storage.operator_role]]
 
@@ -54,6 +52,7 @@ def resource_setup(cls):
         super(ContainerSyncTest, cls).resource_setup()
         cls.containers = []
         cls.objects = []
+        cls.clients = {}
 
         # Default container-server config only allows localhost
         cls.local_ip = '127.0.0.1'