vm/gce differs from other VM types in that it accepts image
in a weird, GCE-specific format (namely, image named disk.raw
is put into .tar.gz file). This makes it impossible to write
generic code that creates images for any VM types.
Make vm/gce accept just image like e.g. vm/qemu
and handle own specifics internally.
The problem with gs:// is that after any filepath.Join
it's transformed to gs:/ which is not recognized.
It's safer to not allow specifying them at all,
rather than constantly breaking them.
Currently gce accepts precreated GCE image name as image config param,
while all other VM types accept local file path as image.
This makes it impossible to write generic code that works with all VM types,
i.e. after building a new image it's unclear if it needs to be uploaded
to GCE or not, and what needs to be passed as image in config.
Eliminate this difference by making gce accept local image file as well.
