Glance supports vhdx disk_format

The disk_format config option enables vhdx as supported by default.

https://docs.openstack.org/releasenotes/glance/newton.html

Change-Id: I783c2f6ca5fe03d5483681611e7c8d1b4dcf9383
diff --git a/tempest/config.py b/tempest/config.py
index c7669b7..ffeeb17 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -505,7 +505,7 @@
                      "users can specify."),
     cfg.ListOpt('disk_formats',
                 default=['ami', 'ari', 'aki', 'vhd', 'vmdk', 'raw', 'qcow2',
-                         'vdi', 'iso'],
+                         'vdi', 'iso', 'vhdx'],
                 help="A list of image's disk formats "
                      "users can specify.")
 ]