image_id is not required to delete a vm from openstack

Since deletion do not check the type of image or anything,
and since that's tedious to keep track of the image_id and
just adding noise to add image_id for nothing, this commit
just relax the requirement.
Merge pull request #3874 from mscherer/relax_nova_compute_argument

image_id is not required to delete a vm from openstack