Changed the disk format from 'iso' to 'raw'

Some clouds does not support other formats except for the raw
which is a default format in the tests now.

Change-Id: I0a308abe2e85df11b411112a3ed7894f99c57807
diff --git a/tests/test_glance.py b/tests/test_glance.py
index 99db954..7313fdd 100644
--- a/tests/test_glance.py
+++ b/tests/test_glance.py
@@ -61,7 +61,7 @@
     try:
         image = openstack_clients.image.images.create(
             name=image_name,
-            disk_format='iso',
+            disk_format='raw',
             container_format='bare')
         logger.info("Created an image {} in Glance.".format(image_name))
     except BaseException as e: