Add vmdk to cinder usage option.
Add vmdk to cinder usage option.

Packstack vmdk option have already supported but there is no
vmdk options at usage.

Change-Id: Ie3e0e047e2fc88ac6295da8a1d6a732fb343f949

fr34k8/packstack,imcsk8/packstack,openstack/packstack,palmer159/openstack-test,openstack/packstack,xbezdick/packstack,mahak/packstack,palmer159/openstack-test,openstack/packstack,xbezdick/packstack,fr34k8/packstack,palmer159/openstack-test,stackforge/packstack,imcsk8/packstack,mahak/packstack,imcsk8/packstack,xbezdick/packstack,fr34k8/packstack,stackforge/packstack,stackforge/packstack,mahak/packstack