Kindly run the needful "packer fix"

[ghost]

Expected: your templates work.
Actual: your templates are out-of-date but it is easy to fix.
Reproducing:

$ packer build -var-file=ubuntu1804.json ubuntu.json
Error: Failed to prepare build: "qemu"

1 error occurred:
	* Deprecated configuration key: 'iso_checksum_type'. Please call `packer fix`
against your template to update your template to be compatible with the current
version of Packer. Visit https://www.packer.io/docs/commands/fix/ for more
detail.

$ packer fix -var-file=ubuntu1804.json ubuntu.json > foo.json
$ mv foo.json ubuntu.json

$ packer build -var-file=ubuntu1804.json ubuntu.json
... skip ahead ...
==> qemu: Gracefully halting virtual machine...
==> qemu: Converting hard drive...
Build 'qemu' finished.

==> Builds finished. The artifacts of successful builds are:
--> qemu: VM files in directory: output-ubuntu1804

[jakobadam]

You're welcome to submit a PR with a fix. I don't have much time.