Reuse a server instance in test_disk_config

In test_disk_config.py, each test creates a server instance everytime.
By applying this patch, a server instance is reused without a new server
creation if the existing server has necessary status.

Change-Id: I1e1f03df3b0d7840b6af666dbb412a7fea2ccbba
Merge "Reuse a server instance in test_disk_config"