Use built in cleanup for servers in test_large_ops

The  scenario base class has code  to properly clean up servers and
wait until they are fully deleted.  Use this instead of custom cleanup
which doesn't wait for delete to complete.

Change-Id: Ic9fe5ea0d267116d5bc78f50d1945602e5247d6a
Merge "Use built in cleanup for servers in test_large_ops"