Add qcow2 image support

This commit adds qcow2 image format for some scenario test. Because
qcow2 disk image format is nicer than uec format.

Change-Id: I4c812176ed1800052c3192110067b1ae7fb04b85
diff --git a/tempest/config.py b/tempest/config.py
index 49d65f9..37c18c8 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -625,6 +625,9 @@
                default='/opt/stack/new/devstack/files/images/'
                'cirros-0.3.1-x86_64-uec',
                help='Directory containing image files'),
+    cfg.StrOpt('qcow2_img_file',
+               default='cirros-0.3.1-x86_64-disk.img',
+               help='QCOW2 image file name'),
     cfg.StrOpt('ami_img_file',
                default='cirros-0.3.1-x86_64-blank.img',
                help='AMI image file name'),