Add ability to skip disk_config tests regardless of extension status

Also, add a missing import in test_create_server

Fixes bug #1089381

Change-Id: I9bc80aca2bec269800fbd20d883596928eedad42
diff --git a/etc/tempest.conf.sample b/etc/tempest.conf.sample
index 020baa1..c4eec66 100644
--- a/etc/tempest.conf.sample
+++ b/etc/tempest.conf.sample
@@ -134,6 +134,11 @@
 # performed, which requires XenServer pools in case of using XS)
 use_block_migration_for_live_migration = false
 
+# By default, rely on the status of the diskConfig extension to
+# decide if to execute disk config tests. When set to false, tests
+# are forced to skip, regardless of the extension status
+disk_config_enabled_override = true
+
 [image]
 # This section contains configuration options used when executing tests
 # against the OpenStack Images API