fix(image_to_vm): Wrap upload_image in vm_upload.

This keeps all references to VM_GENERATED_FILES inside of
build_library/vm_image_util.sh so changes that impact it more obvious.
fix(release_util): Add argument to specify the name to use for .DIGESTS

For multi-file uploads we should explicitly declare what the name of the
.DIGESTS file should be instead of using the first file name. Relying on
the ordering was subtle and easy to break.
fix(vm_image_util): Use more consistent names for vm .DIGESTS

Switch from naming DIGESTS based in disk image name to a common prefix.

old: coreos_production_qemu_image.img.DIGESTS ->
new: coreos_production_qemu.DIGESTS

The old behavior wasn't very consistent since plain disk images aren't
used by all types and the code implementing that was easy to brake,
namely by mistake coreos_production_pxe_image.cpio.gz.DIGESTS became
coreos_production_pxe.vmlinuz.DIGESTS a couple releases ago.

The old names will continue to exist as well for the time being to avoid
breaking existing install/download scripts and the original pxe DIGESTS
name is back.
Merge pull request #162 from marineam/digests

VM image digests