add option "--igonre-checksum" for image-download

When uploading image with VMware vSphere backend, the image will convert to VM template as an image.
In this case, glance image-download operation will export the VM template to a vmdk image file. The VM export operation will
do some format convert, so the hash and checksum values are always different for every image download operation.
So here add an option "--ignore-checksum" and let users to ignore the checksum compare in glance client to avoid error messages.