Use server_client clearly for removing server_group

cls.client is changed on each test class.
The problem 1413486 would happen, because cleanup of server groups
happened in different test class from server group test.
This patch makes cleanup of server groups use server_client clearly
for fixing this problem.

Change-Id: Idfdca6a1fb6a1669e41f0a0484c9f85ece913699
Closes-Bug: #1413486
diff --git a/tempest/api/compute/base.py b/tempest/api/compute/base.py
index 6b6c8e7..c245eb4 100644
--- a/tempest/api/compute/base.py
+++ b/tempest/api/compute/base.py
@@ -174,7 +174,7 @@
         LOG.debug('Clearing server groups: %s', ','.join(cls.server_groups))
         for server_group_id in cls.server_groups:
             try:
-                cls.client.delete_server_group(server_group_id)
+                cls.servers_client.delete_server_group(server_group_id)
             except exceptions.NotFound:
                 # The server-group may have already been deleted which is OK.
                 pass