Use base.delete_server in test_servers.py

In test_servers.py,
1. base.delete_server can be used to replace the multi lines
   of addCleanUp of deleting server and waiting for server termination.
2. wait_until='ACTIVE' can be passed to create_test_server to
   avoid the extra line of waiting for server active.

Change-Id: Ic6e718cf2e3f251abd89692ada0072b29e71e57b
1 file changed