enable xml tests test_disk_config

This enable the xml tests in file test_disk_config.
In diskconfig extension, it require disk_config
must be 'AUTO' or 'MANUAL'. So Change it in
test_create_server.py. It can work before, because
it didn't deal with the parameter disk_config.

Partially implements bp missing-tempest-tests

Change-Id: Ib6ed2f2b4319193ba57a139083e8974deeee3f35
Merge "enable xml tests test_disk_config"