update lxd image disk format for openstack CLI

update lxd image disk format for openstack CLI

JIRA: YARDSTICK-598

As we have replaced 'glance image-create' command with 'openstack image create'
command, we also need to update the disk-format for openstack CLI. the
'root-tar' is no longer a valid choice. (choose from 'ami', 'ari', 'aki',
'vhd', 'vmdk', 'raw', 'qcow2', 'vhdx', 'vdi', 'iso', 'ploop')

Change-Id: I53f839f658c5a6f216f5bb942b1c4f4dcdbf9814
Signed-off-by: JingLu5 <lvjing5@huawei.com>
(cherry picked from commit d64fbf2f276a98a7cc37d92c57f5b7f3517286b8)