Default image in devstack was changed recently from uec to qcow2.
All rally jobs except rally-mos now need to be updated accordingly.
devstack changes:
* Id65ebae73b28da7185cb349b714b659af51ef77f
* I5c154ec25555e768954538fc22b4f5d5975b2deb
Change-Id: Ia4d4afb661d8bcc7c38b0a16b508294f789eef14