Change testcases 'cirros' image to 'yardstick-image'

Change testcases 'cirros' image to 'yardstick-image'

The default OpenStack 'cirros-0.3.5' image doesn't exist anymore. The
image included by default in any testing OpenStack deployment is now
called 'cirros-0.3.5-x86_64-disk', but this name can change.

Because Yardstick, during the installation, deploys a customized image,
'yardstick-image', it makes sense to use it as the default image.

JIRA: YARDSTICK-873

Change-Id: I8fd7cbc01d4fa52e9107f2fac7d2d9ff5d1091c7
Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>