Bump version of cirros

This is an attempt to workaround an issue with insecure key-exchange
algorithms:

https://www.openssh.com/legacy.html

I'm hoping that the version of dropbear has been updated,
fixing this issue.

Author: jovial

bin/rally-verify-wrapper.sh

@@ -72,6 +72,7 @@ crudini --set ~/.rally/rally.conf openstack flavor_ref_ram 128
 crudini --set ~/.rally/rally.conf openstack flavor_ref_alt_ram 256
 crudini --set ~/.rally/rally.conf openstack flavor_ref_disk 1
 crudini --set ~/.rally/rally.conf openstack flavor_ref_alt_disk 1
+crudini --set ~/.rally/rally.conf openstack img_url http://download.cirros-cloud.net/0.5.1/cirros-0.5.1-x86_64-disk.img
 
 rally deployment create --fromenv --name openstack