DevStack Ocata version and master use different default images
(Ocata:uec and master:qcow2), this will lead to tempest encrypt test
failure in grenade gate.
This patch hard-code default images in base version and will be
removed if devstack master and ocata patches are proposed.
Change-Id: I997c759fc026366fe48de9ac7e8c58941622c9cd
Co-Authored-By: Nam Nguyen Hoai <namnh@vn.fujitsu.com>
f6489d8355