Trivialfix for help description of images_type
Trivialfix for help description of images_type

The file virt.py has been remove from the patch
https://review.openstack.org/#/c/392566/.
So use_cow_images config is now in file compute.py
Change from virt to compute for it.

Change-Id: Id332f6dc3e0aaaab4ff94b810e4a5bf6b7e01874

openstack/nova,klmitch/nova,rahulunair/nova,rahulunair/nova,openstack/nova,klmitch/nova,mahak/nova,mahak/nova,mahak/nova,klmitch/nova,klmitch/nova,rahulunair/nova,openstack/nova