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
1 file changed