Fix web-download trying to import qcow2 as raw

This test aims to specifically import a qcow2 image via web-download,
but it creates the image with no such specificity. When glance begins
to inspect the stream, this will fail checking.

Change-Id: Ie901e61a1d2b395ffaff47aabbd9d3840c6b0327
diff --git a/tempest/api/image/v2/test_images.py b/tempest/api/image/v2/test_images.py
index f6ca317..e9c7dba 100644
--- a/tempest/api/image/v2/test_images.py
+++ b/tempest/api/image/v2/test_images.py
@@ -132,7 +132,7 @@
         """
         self._require_import_method('web-download')
 
-        image = self._create_image()
+        image = self._create_image(disk_format='qcow2')
         # Now try to get image details
         body = self.client.show_image(image['id'])
         self.assertEqual(image['id'], body['id'])