add an option for nova to inherit the instance disk format from glance image

currently we set the format for all instances in images_type under nova.conf:
# VM Images format. Acceptable values are: raw, qcow2,
# lvm,rbd, default. If default is specified, then
# use_cow_images flag is used instead of this one. (string
# value)
#libvirt_images_type=default

I think that since some environments can support more than one type it would be a good addition to add an option for instance disk type to be set by the glance image format.